Tri riders have the saddle almost directly above the BB, this allows makes it more comfortable when they ride aero, and puts less strain on the muscles they use when running. Having the BB further back means less space for a rear wheel. I think it is mostly habit and ease of parts that means people use 700C wheels. Check out the research by Alex Moulton on wheel size, he made a 20" bike that I think still holds the speed record for riding in an upright position.

Smaller wheels are lighter, but you have to spin them faster so I don't think there is any real difference in acceleration/speed, the conclusions from alex moulton is that tyre construction and pressure makes a much bigger difference to speed and there are just more tyre options for 700C bikes.