Part of it is the lack of guns- we don't have revolvers and lever actions. It's just not Western
unless you have a pair of low-slung six-shooters.
But a big part of it is that "Western" isn't a thing at the moment. It's not "trendy", like, as above, the "modern warfare", "streets of Fallujah" thing, or zombies. And I mean not just paintball, but in pop culture in general- what was the last decent western movie we had? Cowboys and Aliens? Even Wyatt Earp and Tombstone were twenty years ago.
We talked about the same thing over on the Guild not too long ago, specifically that there were very few western-themed webcomics. There's about a hundred "guys in college" comics, and a hundred more "guys in an office" comics, space themes, D&D/WoW themes... but I think we only came up with two "western" themed strips, both of which were more sci-fi/steampunk than western.
One of these days we'll get a good revolver or lever action (I have stacks of drawings, anybody want to commission one?
) and it'll take off, even if just as a trend for a few years.