They are transplants from the ATB world, which nearly took over biking in the pre triathlon/pre Lance era.
In the early '90s ATB were outselling roadbikes 9:1. Manufactureres discovered that slant top tube bikes
were easier to stock as 3-4 sizes cover the market, where parallel top tube frames needed 6-8 sizes to cover the market. The threadless fork is one size fits all frames, just chop the head tube to size and use a few spacers to fit it to the bike, whereas threaded forks must be very close to the right length and shortening one risks buggering the threads hence are made to fit the frame So now you know why slant top tube frames are taking over the road bike world: so shops and frame makers can get by with smaller numbers of frame sizes and simplify their stocking.

Of course it helps that riders buy a lot of them as well. I will eventually get used to their look, but still
prefer the classic DF, parallel top tube bike. Slant top tube makes me think of gnus.
Steve