The most natural, direct response—as far as I can tell—is #6: "If you found it, why didn't you hand it over to the police?"
I don't believe any of them are grammatically impossible. The other options are legitimate sentences but (like so many aspects of the English language) they have subtleties that alter the meaning slightly. For instance:
"If you found it, why wouldn't you have handed it over to the police?" – the asker is possibly asking about a situation that happened to someone else, and are asking why you would have resolved it differently to that person. In this case, someone else found the wallet and did hand it in, and you have expressed that, if you were in that situation, you would not have done so. The asker is asking why you would choose the opposite action, and there is an implication that they are surprised or disappointed by (or do not understand) your motives for keeping the wallet.
"If you found it, why shouldn't you have handed it over to the police?" – this is asking again in a hypothetical situation, but it suggests that you did hand it over to the police and asks if there is any reason that you can think of, why that might not have been the only reasonable action for a person in a similar situation. It does not necessarily infer the surprise or lack of understanding of motives (that would depend on the tone of voice of the asker), it just asks what motives you can think of.
"If you found it, why wouldn't you hand it over to the police?" – again, a hypothetical situation, but one where you find the wallet and don't hand it over; it includes the implied suprise/lack of understanding but the asker is concentrating on why handing the wallet into the police is not the only reasonable action for you, specifically. In casual spoken English, this might be used in place of #1, as it feels more natural to speak, and there would be differences in emphasis (e.g. on the first you) which would determine whether the asker meant the same as #1 or not.
"If you found it, why hadn't you have handed it over to the police?" – there's a past-tense, second-person flourish to this wording, that feels more like a sentence you would read in fictional writing, where the author is describing their surprise or disappointment of the actions of their narrator character, rather than a direct question to you in response to an event. More-common variations of this exist in a first-person narrative, when the narrator questions their own motives ("I found it, why hadn't I handed it over?") and in a third-person perspective, where a character is questioning the motives of another ("If he found it, why hadn't he handed it over?"). Of all of them, this is the one that feels the most grammatically awkward (but not necessarily wrong, in certain contexts). That said, the construction, "Why hadn't you have handed it over" isn't one I would use—but I'm not sure if the extra have is deliberate in your question, or a typing error. If not a typing error, then I'd say "hadn't you have" was grammatically incorrect, in favour of "If you found it, why hadn't you handed it over...?"
"If you found it, why haven't you have handed it over to the police?" – this one suggests that you still have the wallet, and that you may (or may not) intend to hand it over to the police, and the asker is is asking why you haven't done that yet. Again, the construction, "haven't you have" isn't one I would use—I'd simply say "If you found it, why haven't you handed it over...?". If the extra have was not a typing error, I would say this one is grammatically incorrect, also.
"If you found it, why didn't you hand it over to the police?" – this one best-answers the brief. It implies that you did find it and didn't hand it over, and the asker is asking why you did not. Whether or not the asker is surprised or disappointed is dependent on the tone of voice the asker uses, but it simply states two facts and asks for the reason without any narrative flourishes, so it is the one I would choose in most cases.
Comment: I felt strong doubts about ##3,4. I should have written: "...why hadn't (haven't) you handed it over to the police". I think that in "...hadn't you have handed", the verb "have" looks grammatically impossible, doesn't it?
The sentence in #3 is fine, if that is what you intended to say. Re-reading #4 and #5, I now see the extra have in those sentences. I had originally considered them typing errors (which most native English readers will ignore when reading casually). If the extra haves were intentionally included in your sentences then, yes, I would agree that these were grammatically incorrect. Without the extra haves, those sentences are valid, at least in the forms I have tried to describe.
Comment: I think I've gripped the meaning of all above versions but that of #1. I'll try to mull it over and adduce my way of understanding of #1
Sentences #1 and #2 are similar to each other, but may need an alternative examples to help clarify between those and #3. Consider if your sentence we part of a conversation about someone else handing in a wallet. The conversation may go:
Friend: John found a wallet and handed it over to the police
You: I wouldn't have done that, if I found it.
Friend: If you found it, why wouldn't you have handed it over to the police?
You: I am poor and need the money, so I wouldn't have handed it over. I would have kept it.
In this case, you have suggested that, in the same situation as John, you would have done differently. The question in #1 asks why you would have done differently.
In the case of #2, you don't necessarily need to be talking about a second person (and you might change I to he if you were). Sticking strictly to the wording of option #2:
You I found a wallet and handed it over to the police. I shouldn't have done that.
Friend: If you found it, why shouldn't you have handed it over to the police?
You: I am poor and need the money. I should have kept it.
In this case, you have suggested that you regret the action that you took, and the question in #2 asks why that might be.
Comment: From #1...Shouldn't we say:"If you had found a wallet, why wouldn't you have handed it to the police?"
I'd agree that the "If you had found..." phrasing is more correct (certainly when written down) but, as a listener, I'd accept the omission and fill in the missing had, subconciously, in the same way that I'd relate it to the wallet.
However, depending on the verbal stress that you can place on either you or had, you can change the implication of the sentence again:
- "If you had found the wallet, why wouldn't you have handed it over..." emphasises that the question is about why you would have done differently, given that someone else did find the wallet.
- "If you had found the wallet, why wouldn't you have handed it over..." would suggest that you were looking for the wallet and you didn't find it, but that your intention was to keep it anyway. The question is asking why you wanted to keep it.
Even the construction, "If you found it", as originally written, can also have different implications, depending on where the vocal stress is placed:
- Stressing the if reinforces the hypothetical nature of the question
- Stressing the you reinforces the fact that someone else already did
- Stressing the found reinforces both that you did find it and that the asker expected you to hand it in
- (Stressing the it may suggest that you don't know what the subject is, or that it was abstract, but that isn't the case here)
Comment: And what if we are trying to convey the idea of reality in the if-clause?
I might use the alternative since, instead of if, if I were emphasising that something really did happen, i.e. "Since you found it, why didn't you hand it in.". Since is effectively the same as Given that, and there is no longer any ambiguity as to whether or not something happened.
If I were trying to convey that both the reality and the time delay was significant to my question, I'd use when; i.e. "When you found it, why didn't you hand it in?"
Comment: Can our initial main clause ("...why wouldn't you have handed it over to the police?) equal the meaning of "...why wouldn't it have occured to you to go up to the police and hand the wallet in?"
Yes, the two statements can be equal, but it may still depend on verbal stress. Consider also:
- "Why wouldn't you hand it over..."
- "Why wouldn't it occur to you to hand it over..."
- "Why didn't it occur to you to hand it over..."
There is an element of incredulity implied by the verbal stress—that any "normal" person would have handed it over to the police. All three of these may be framed as an attempt by the asker to coax or shame you into admitting that you knew what you were doing was wrong – perhaps as part of an interrogation.
If I were unambiguously trying to inform someone of the correct procedure, rather than determine or comment upon their morals (e.g. a part of an admonishment), I'd probably say "When you found it, you should have handed it into the police."