Translation:Why didn't you tell me?
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. :)
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.