*sigh*
The bottom line is that while graphically superior, many other engines just lack a LOT of things that half life has. IMHO, theer hasn't been a really really good modding engine since Half Life. This is for several reasons.
1. Half Life mods, especially the good ones, have a lot of support from Valve. If you have a question that needs answering, usually Valve will try to help if they have time. Furthermore, play your cards right and the source can become available to you. Looking at UT2K3, I heard it was hideously expensive to liscence the engine, even for a single programmer.
2. Many people are very used to Half Life editing, and there's a slew of exceptionally good tools available. Furthermore, an engine switch would mean trashig everything, even textures probably.
3. You can't name ANY game that has the fanbase that Half Life has. This mod wouldn't be doing nearly as well on another engine.
4. Half Life runs very nice and smooth on even older computers. It'd suck if the best we could do was look at screenshots. I bet most ppl here would be hard pressed to run a game that really pushes the UT2K3 engine. I don't even think I could run UT2K3 at more than a few FPS.
All in all, even now, I'd say the HL engine is still the best modding engine. Sure, it doesn't always looks as good as other engines, it can't handle super high poyl mdoels, or rediculously huge maps, but in the end, does that really matter? Look at some of the mods for UT2K3. A lot of them REALLY suck. Moreso than some HL mods. I'd rather play a silly mod like Rocket Crowbar (oh god it's fun, too bad there's no servers - but if anyone wants to play, I'm up for it
) than most UT2K3 mods.
Anyway, in closing, HL rocks for modding. ESF rocks HL.
-Karrde-