Dan Empfield designed the first tri-specific bike. Since, in the original triathlons, there was no drafting, he was basically designing a bike frame around a position that would work well with aerobars. Dan had to invent stack and reach measurements in order to classify and compare bike frames, and in order to get the right kind of aerobar position, he wanted to bring the saddle position forward. That's a long introduction to why the seat tube angle on TT and Tri bikes is so much steeper than on traditional road bikes: it's to bring the saddle forward. (A related early design was dual purpose road/TT bikes, like the Cervelo Dual and Cervelo Soloist, which had reversible seat posts that would make the "effective" seat tube angle steeper). Steve Hed saw what Dan was doing and pointed out that the riding position looked like positions he'd seen for recumbent racing, only rotated clockwise around the BB. Anyway, that's why there are limitations on how far forward the tips of your hands can be: without those limitations, racers would be even more rotated and forward.

(BTW, frame stack and reach depend on the fork which (traditionally) isn't part of the frame (that's why we say "frame and fork" instead of just "frame."))