The original publisher of the games, Sierra Entertainment went defunct in 2008. The developer of the most recent PC variant of SWAT 4, Irrational Games officially closed shop last month ending any hope of either an expansion pack to follow-up The Stetchkov Syndicate or a modernized and updated version of the game, such as a SWAT 5. Given the success of the Bioshock franchise, it is easy to see why SWAT was dropped from their efforts.
With the closure of Irrational Games, the franchise is technically owned by Activision Blizzard at the moment, and there are no known plans to make another SWAT game. That company stated in 2008 that various titles were jettisoned for not exhibiting "potential to be exploited every year across every platform".
Translation: if we can't shovel ware it down your throat each year with a $49.99 patch that can be played on every device imaginable, we aren't making it.
I briefly covered this series when I had it on my XP gaming rig and those posts can be accessed here.