unlike hockey, there is a strong grassroots soccer in pretty much any part of North America. MLS's main financial goal is to get that big money TV contract, and to do that, you need viewership all over America.
and unlike hockey, I think it's easier for 'small' market teams with good ownership to be successful in MLS.
I don't see any other city having a 2nd team, until MLS gets to sustainable pro/rel, with good number of teams to support it.