What I wrote in the title is my question~ Is there any available explanation for why 'crooked' is pronounced as 'krʊk/ɪd/' not 'krʊk/d/? Or is it just one of the exceptions for '-ed' ending words and it is what it is?
There is a comprehensive answer to this question on the English Language & Usage sister site.
Basically, the answer boils down to the fact that the adjective "crooked" is not derived from a verb, and so it doesn't follow the typical -ed pronunciation rules for past participles. On the other hand, "to crook" is a verb and has a past participle "crooked," as in "He crooked his finger." This word, although spelled the same as the adjective, is pronounced as a single syllable, like "looked."
This same situation also arises in some other words like "blessed."