One downside is that if (when) the equipment fails, it'll be unfair to whoever happened to be lumped with the bad one and it would sometimes be hard to prove if it was the equipment's fault or the athlete's. With their own bike, it's their own responsibility.

Another is there would be no pressure on the bike technology so the audience would know they're not performing as well as they could on some consumer bike, reducing the appeal of it. It's already stink that there are weight minimums.