San Antonio is a bigger TV market. The Rangers are blacked out for the most part in San Antonio, so it would be the Astros who would hypothetically block it. Cowboys and Texans have had the same problem for ages because teams will relocated to San Antonio because of the TV Market size. This is why Angelos owns the Nats' TV rights, because he could have sued MLB from putting a team in DC by the leagues own bylaws. This is also why good ol' Jerry Jones helped Kronkie, Davis and Spanos move their teams to LA and Vegas. That and with MLB, I think broadcast range is for exclusivity is around 100-150 miles, maybe? I can't remember, I'll ask dad tonight.
Dallas is 5 1/2 hours away from San Antonio with 274 miles in between the two, with Houston being about an hour less away with 197 miles between the two. Portland is about 3 hours away, with seattle 173.0 miles away. The TV markets are what honestly will decide who gets a team. The Rangers and Astros can't put up a fight like the O's did years ago due to distance and market size. Dallas and Houston are the 5th and 10th biggest markets in the US, so losing San Antonio isn't that big of a deal. San Antonio's is 37th nationally, and the team would hypothetically have exclusive rights to Austin as well, the 49th largest market. Portland is 25th by itself, but the two markets merged together would have greater pull than Portland. Seattle is the only team in the pacific northwest, and I have no idea how desperate the Mariners would be hypotheically to defend this market.
It appears you contradicted yourself by saying "S.A. is a bigger TV market," then mentioning that it's smaller than Portland (which it is, though S.A. is higher than 37th).
Also Rangers games, via Fox Sports Southwest, ARE carried in San Antonio. Though given the business model in which MLB teams generate local TV revenue, as long as FSSW would still be carried on S.A. cable systems with a team there (and these things are contractually negotiated over a course of years) then the losses aren't as big because the revenues are baked-in to every customers' cable bill. Same goes for Astros games in San Antonio, and Mariners games in Portland.
I agree that Baltimore and Washington really aren't a good comparison due to their proximity to each other compared to the above examples. While expansion may lead to some compensation for an owner whose territory is infringed upon by a new team, I'd almost guarantee it won't be on a level near what Angelos got....MLB doesn't want to make that mistake again.
Back to my main point, I can honestly think of several reasons why Portland is probably not the best candidate for MLB expansion, but you haven't brought up any of them. It has nothing to do with the TV market or metro area being significantly smaller than the other cities, or being too close to Seattle, or that the Mariners would be able to successfully form a bloc if MLB deemed Portland it's best option.