Recent trends in the attendance numbers show some gain in New England, Columbus and DC. The biggest markets, however, still lag far behind the Pacific Northwest, newer league teams and recently opened stadiums. Here we speak of LA and New York. LA has been stagnant mainly due to poor form while New York just can't seem to get their city excited about their team no matter who they hire to play. Example: midweek fixtures in New York (13919, 55%), Chicago (11,947, 60%), LA (16,512, 61%) compared against Seattle (38,311, 100%) show the discrepancy.
It is true that these major markets have other major sports, but you don't see their numbers suffering. Not too long ago, LA sold out every match, regardless of how full the Staples Center was. It comes down to the product on the pitch (LA, Chicago) and, well, the New York fickle crowd. Is a second team in the city necessary? Would it make a difference? Only time will tell.