In road racing there are many limitation put on the bike by the sanctioning body in Europe. They want the Race bike to "look" a certain way. Aero tubing is significantly limited and for many years the top tube had to be level. "They did not like the Look" of sloping top tubes. In Triathalons it is kinda like if it has two wheels (I think 3 are legal too) and it rolls under the power of the rider then it is legal. Drafting is illegal in Triathalons so the small wheel benefit there is not relevant.