I think the logic of the original question may be off a bit: Why do NWN modders avoid NWN2? Is it avoiding NWN2 to mod NWN instead (or any other game, for that matter)? How many people mod (to a pretty great extent) more than a single game? The fact (disputable and disputed, obviously) that the two games are similar could, if anything, be seen as more reason for someone not to mod both. Why mod one when you're already modding the other?
I think the question is really:
Why don't NWN modders stop modding NWN and instead mod NWN2?
See, if it's phrased like that, it sounds a bit different. BUT!!!! I could be wrong about the OP's intent with the question.
As for the quality of NWN2, I think it's a fun enough game to play, once it's patched up and you have a capable rig. The controlling of PC movement, coupled with the camera focus, is not my favorite thing. But I enjoyed a few modules I played. From a toolset perspective, the toolset doesn't seem hard to learn, in the same way that learning to apply paint to a canvas doesn't seem hard to learn. One can learn the fundamental motions, swiftly enough, but that doesn't make one a skilled artist.
I think NWN2 is an easel. It can create some beautiful 3d environments that are a pleasure. But to me, it feels more like looking at a beautiful painting and less like wandering through a world. Area transitions are jarring and kick me straight out of any feeling of being in a world. Movement seems disconnected from the environment. Worst of all to me though, is that however pretty it might look, I always felt like I was trapped in a narrow channel -- a very sweet looking narrow channel, bumping up against invisible walls that stopped me from going over to look at a rock that was just off the trail. NWN feels bigger, more open, and thus (to me) more real.
To each his own, and I sincerely wish everyone playing or modding either or both games to HAVE FUN doing it. Games = fun.
Regards,
JFK