Why is Detective Munch, of Law and Order: SVU, not a primary on a case.?

When SVU started, "Munch" was a supporting actor, as well as everyone else on the show were supporting actors and actress'. They said that everyone was the star of the show. Then that kind of changed and now it seems "Stabler and Benson" are really the stars of the show and everyone else is a supporting actor/actress. Unfortunately that happens a lot when shows have been on the air for quite a while. There are sometimes an episode or two where Munch is one of the main detectives, but it's usually like 2 or 3 episodes a season.