I'm glad everyone had such fun with my four reasons.
I guess if they'd been funnier they would have been taken less seriously.
But now the OP hasn't returned to give his/her actual reason for the preference. And the OP is carfree and carefree, which I dig.
My bad.
As penance, I'll try to think of some actual reasons for wanting a non-slopey top tube.

1. More space for a shoulder carry cyclo-cross style.
2. Proper tube angle for one of those inside-the-frame bags.
3. In the case of smaller frames, enough room for a full sized water bottle in the seat tube cage.
4. Old school aesthetics.

How's that?