San Antonio has only 1 pro team at the moment, is the 7th biggest city (37th size market, because people don't live outside the SA borders). That's only 6 spots behind Kansas City, which hosts more sports teams. SA's support for the Spurs is very healthy. So I don't know why they couldn't host one.
I'm not sure where El Paso is on the list, but good luck getting any players who have a choice to go to El Paso. There are at least 20 cities I'd choose before El Paso.
El Paso:
I don't think the location of the team really plays a factor into who plays on the team. If the team has a good ownership group, a good coach, and solid fan support from the local community I can't imagine players turning down the opportunity to play for that team.
Also, I don't see why the perception of El Paso is so awful. It has a largely hispanic population that would eagerly support the team. There are also the "affluent" young white people who would gladly support the team as well. Having said this, I don't think that the El Paso market would draw in a bunch of fans who weren't from the West Texas, New Mexico, and Northern Mexico area that it already influences.
If I was pushing for a team in El Paso I'd try and get an NASL or USL Pro team first.
San Antonio:
The healthy support for the Spurs can largely be attributed to the fact that the team has kept the core of its roster the same since the drafting of Tim Duncan and the continued success that it has every NBA season. If the team had sub par successs you wouldn't see the same level of support that you do today. I honestly do not believe that San Antonio can support another professional sports team of any variety.
San Antonio isn't exactly a choice destination for professional athletes. However, I don't think that the city really affects the players that the team is able to sign.
Miami:
Miami is one of the worst major sports towns in the United States.
I really hope that David Beckham gives it up and moves on from Miami. There are plenty of other deserving cities that he could try and get a team in.
Detroit:
I really doubt that Detroit would choose to support a soccer team. (Just a gut feeling)
The city is also a complete and total piece of crap. (My sincerest apologies to anyone from Detroit)