Why is that? Wouldn't a little competition between manufacturers be interesting to follow? Certainly car racing shows results based on manufacturer.
The rule is:
F.5. RANKING On events sailed under F18 rules, there shall not be specific rankings according to the different competing brands or types.
I have no idea why this was written into the rules, so your guesses are as good as mine. I think the rules were written to try to attract as many manufacturers as possible but still end up with a class where "the nut on the end of the tiller" is still the most important component. It seems to be working from what I have observed in the last few years. It seems to me that the best sailors, regardless of boat, are rising to the top in all competitions.