Because Q&A answers are contributed content. Once they are posted, the maintainers of the section can edit or delete them at will, or combine several answers into one, etc. Given this, it doesn't make sense to show the replies which may no longer be relevant. They are still visible when looking at an individual answer, because normally you would be looking at a question and it's answers together.
(That's my guess, or at any rate an argument for not changing it. It's equally likely to have originally been a bug.)