Rotten Tomatoes focuses exclusively on English language reviews from "approved critics"(http://www.rottentomatoes.com/help_desk/critics.php ), so it's not that much of a surprise that there aren't that many Japanese anime films rated there. Since the 90's, the only major theatrical releases in the US have been Studio Ghibli films, and that's because Disney handles the US distribution.

I would imagine that it's similar for the vast majority of foreign films as well.