A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

And now you're sitting here wondering what possible relevance that might have
to some line or other - the only context you have at this point is that it's a
reply to something you wrote. Actually, at this point you don't even have that.

So you may have read this entire thing and now you're still wondering what
possible relevance it may have to the thread.

On Tue, 15 Jan 2019 00:24:30 -0500, bzs@theworld.com said:
> Why dig through what you've already read to see the new comments?

Or you can put the comment after, so everybody who reads text top to bottom has
the context.  I'm not away of any languages or writing systems that work from
bottom to top, so that's pretty much everybody.  And if people trimmed the
quoted material so only the parts being replied to are left, there's not much
digging involved.