Huge markets. Seattle isn't huge.
If market size were no issue, you'd have teams in Columbus, Buffalo and Indianapolis.
If you want a huge market, try Mexico City. We’ve already got a team in Canada—why not Mexico. Half the players are latino anyway.
Hell, the Mexicans are slowly taking over our country anyway.
Portland, Oregon is the largest urban area in the US w/o a major league team.
http://en.wikipedia.org/wiki/Table_of_United_States_Metropolitan_Statistical_Areas