Hi qwerto,
The answer is fairly straightforward - it is because if the race featured the trimarans of your example then it wouldn't be a monohull race
The aim is not simply to go as fast as is possible using any possible format. The aim is to extract the most speed out of a particular format. Otherwise, we'd give up sails, bolt rockets onto those 60' trimarans and go even faster!
Of course, nothing prevents a bunch of 60' trimarans getting together to race each other (would be pretty exciting stuff, really).
