Is Why not more? correct, or should it be Why no more? What does it mean, then?
Why don't you want anymore of something?
Or the opposite:
Why not having some more of something?
Note that neither of these phrases is a full sentence. Both are responses to something that has been stated. Exactly what they mean depends on the context, so one can only give examples of their possible use.
A: I'm afraid that I can only offer you less than you require.
B: Why not more?
So B is asking why it is not possible to obtain more of the product concerned.
A: No fuel supplies will be available from tomorrow.
B: Why no more?
Here B's response could mean either why will there be no further fuel supplies or why will they no longer be available.
It's a question of context.