Someone mentioned market pressure and that's part of it. Most custom builders say the overwhelming majority of their customers pick a sloping top tube given a choice. Many non-custom frames come only with a sloping tube so many people obviously like it. Several of these manufactures did extensive market research first and customers like the looks, function, and aesthetics of a sloping tube.
In terms of functions, the sloping tube serves several purposes. It allows some configurations to be set up so the frame doesn't look funny. The compact design allows for the bars to get higher if that's what the rider wants. It also alllows for a slightly stiffer bottom and rear. Finally it results in a slight weight reduction, particulary if a super long seatpost isn't required.