I'm not a fan of soccer, but I can see the appeal. I think it doesn't do well here in America because there isn't enough stoppage in play for networks to run 20 beer commercials or 10 Pizza Hut commercials.
I think that if MLS was broadcasted and covered like other sports that it would be popular in the U.S. maybe not like it is in Brazil or England but there are quite a few soccer fans.