"Why haven't they replied yet?"

Translation: 为什么他们还没有回复？

[Semeltin]

Would 回答 work?

[King184867]

回答 means answer, and 回复 means reply.

Searched from the web:
Both "Answer" (回答) and "Reply" (回复) have the same general meaning and can be chosen between. Though they do have a slight difference; "reply" meaning that you respond to someone making conversation with you whilst "Answer" is when you respond to someone's question.

The better one here is 回复.

[RobinThor]

Why is 为什么他们还没回复过 wrong?

[rabbitcca]

because ”为什么他们还没回复过” translates to something like "why haven't they replied before".

[Carissa923906]

Because that's like saying "why have they not replied already/before"