I respectfully disagree. Well, perhaps so for people who like adversarial games but for cooperative gamers Swat 3 is much better.
Just a wild stab in the dark but perhaps the reason why they don't sell it is because of license issues. Swat 3 was fully developed as a Sierra game while Swat 4 was a Vivendi-Universal game. However you would expect that when Activision bought the franchise all of the games would be under their control.
Maybe because Activision still views it as a "newer" game and still has value to them in case they want to continue the series?
Activision doesn't sell it on their website as far as I can tell and I don't think a digital download is available anywhere (legally).