Because its possible that someone could win 4 categories, and get all of the prizes (except 2 which they could theoretically also get one of), which is debatable over whether it is very fair that they win them all. The best map winning makes sense, but it would cut out slightly less skilled mappers, as one very good mapper probably could make a map that would win all of them. Then maps which still are good but not quite as good get no publicity and the mappers who worked hard on them get cut out for just about not being good enough.If it really is the best in more than one categories, why not?