I guess to stop anybody from going that route and build really expensive boats.
If Marstrøm or someone else buildt some F-18's from pre-preg S-glass, put som olympic pros on it and they won the worlds and europeans, dont you think most sailors would want one of those? This is exactly what happened in the Tornado class, and as a result it's as good as a one-manufacturer class today.
I think it is a wise move for the F-18 class to forbid pre-pregs in the hulls. If they allowed carbon in the hulls, I would have a different opinion, but then they should lower the max weight also ( and morph into the 18HT, lets not go there today..).
Also, the technology for going into pre-preg production is expensive. Autoclaves are not cheap..