Here are my two main reasons, which I'll expand on when I have the time:
1. Television market. Right now, Seattle has a decent chance at being succesful over the long term because they have a monopoly over a mid to larger sized television market. If you split that market with Portland, the Mariners would be left with a much smaller tv market, lessening their revenues drastically, and putting both the Mariners and the Portland MLB team in Milwaukee land.
2. No one seems to want to argue for a Mexican based MLB team. There have been two teams founded in Canada, and there have been some MLB road games played in Latin America. From a strickly economic point of view, putting a team in say Monterrey, Mexico makes sense. Three million people, highest quality of life of any major Mexican City, and close to other MLB markets.