没有 is used for past tense negation, while 不 is used for present (and future), so "why don't you tell me" would be "为什么你不告诉我" (I'm not a native speaker though so someone please correct me if there's any other issue with that sentence!) https://resources.allsetlearning.com/chinese/grammar/Bu_and_mei(you) lays it out nicely with some more examples. :)

no, because the 没有 indicates past tense

Context

Because that is incorrect word order in English. In a simple sentence that starts with a question word and only has one clause, the first verb always comes immediately after the question word.

I'm sorry, but this comment just made me laugh so hard. Reread what you wrote in a Chinese accent