Because they work with a old version of the engine (they call the source engine right?), the halflife latest one was tweak and have the support of the creators (valve)...
I know the game have serius problems, it take me buy a 512 mb RAM for my computer, because the game not run with the 512 ones i have in that moment, still with 1 gb ram i have slow downs, and the 1500 mb virtual memory that they recomend in the readme was insane.
Damn it was like Ultima 9 when it come out, no one play the game in the time it get release because it have some problems with the TNT video cards and it ask for 256 mb ram ( in that time that was too weird have that amount of ram).
After some patch and some years, i play the game that was in my shelve, was nice, of course not near as the ultima 7....
Soo my last favorite company die, i really don