Back in the day when MTBs started getting sloping top tubes a lot of makers only sloped them for smaller frames. Back then you wanted a higher seat cluster, though, which allowed for higher canti bosses, which reduced heel/brake arm strike. This made horiz top tubes a plus for larger MTB frames.

Then there were a couple of years of UBrakes, then lower profile Canti arms, then V-brakes allowed the super slope and discs do as well.