It's just because with alot of FPS on DS they use the directions to move and stylus to aim, you hold the stylus in your right hand, which is cumbersome unless your ampidextrous. They do have a left handed option with most games but in it you use the A/B/X/Y buttons to move and that just sucks. Though if you have a thumb strap it can make things alot esier, even if brothers in arms isn't designed for one (e.g. throughing granades made hard) still ist's a decent game.
Thier really arn't many at the moment, call of duty was great while it lasted, but ended to quick and that was it, Check into a game called C.O.R.E thats hopefully comming out soon it looks pretty oggd, a bit like the original Quake.