Is it down to the popularity of the FR settings when the games were released, the plot, the combat, the storyline, the offbeat characters, the humour ? All of the above? Something I missed?
When NWN was released I expected the BG community to transfer. Why are people still working with this old engine when a newer, designed for modification engine exists?
There's no question that it's puzzling. But BG and BG2 have simply proven themselves to be eminently (re-re-re-re-)replayable. This encourages modders to keep working, because there are still people interested in "just one more go, but this time, with X, Y, and Z added in."
And for modders, the games are such rich landscapes that it's tempting to say, "Oh, but what if THIS character showed up?" or "What if we were able to TRIPLE-cross that guy?" or "What if the 'when did I eats corn?!' beggar really DOES have the hands of a surgeon?"
Although there's no question the D&D/FR aspect helped make BG2 a huge seller, I don't think it has much directly to do with its longevity. (Indirectly, of course, it's easier to still have a sizable audience after 8 years if you START with an enormous audience.)
I don't absolutely know for certain that virtually no Infinity modders have transitioned to NWN (although it certainly appears that way), but if I had to guess why, it would be for some combination of the following reasons:
1. As mentioned, the NWN core materials tend to be less compelling to mod than the BG2 core materials, which means you're more likely to have to "do your own game." But because "do your own game" was/is so much harder for Infinity, Infinity modders tend not to think in those terms as often, don't have the skillsets or the interest or whatever.
2. The scripting and dialogue skillsets don't translate over terribly well. NWScript acts a lot more like a "real" programming language, so people who can shuffle by with the crude but easy-to-follow Infinity scripting standards could be lost. And there's no easy substitute for WeiDU, meaning complex dialogue creation is a lot harder and a lot different.
3. The "modding scene" operates very differently for NWN than it does for Infinity. We're pretty much all working with the same independently-operated portal-and-forum model TeamBG had back in 2001, but NWN is much more driven by the massive, professionally-managed community sites. From an outsider's perspective, it's a very different vibe, and I suspect not as inviting for people accustomed to the Infinity scene.