I have created a number of NWN mods and I've been working on an NWN2 ever since I opened the game box. I think there are a few reasons.
1) When NWN came out that toolset was unique. No other CRPG gave you the flexibility to actually create your own "games". Most toolsets allowed you to modify the OC and add your own elements to the game as it was shipped. But with NWN, you never neven needed to play the OC. Now it seems the novelty has worn off a little.
2) NWN now has 5 years, two expansions, and a ton of community made tools. NWN2 has a handfull of user made tools, a very limited Monster selection, and no expansions. And has been out for less than a year. The first expansion will soon be out and may well add enough content to kick start the modding community. The NWN modding really did not hit it's stride until after SoU and the first CEP was released. So it is a little early to call it dead. Granted there were some really good pre-SoU mods, but the best came after. Another thing to note, by using Granny, Obsidian made it difficult to create custom content like new monsters and animations, tiles, etc. With NWN it was not easy, but with NWMax and a few tutorials anyone could do it. That is not a criticisim of Obsidian, I know and understand the why behind the decision. Some things cannot be helped.
3)The tileset system in NWN made it very easy for a rank amatuer to create a decent looking area in a short time. The trade off was, it did no look that great and became horribly repetitive. The NWN2 height mapping is... extremely detail intensive. It is not impossible to use by any means, but it takes a hell of a lot more effort than the NWN Tilesets. The results when done right are really, really impressive. And when done wrong, extremely discouraging to the modder. When it comes to placing objects, triggers, etc it is really not much different from NWN. Scripting of course is the same. But making a good looking playing area requires a lot of thought and effort. And it takes a lot longer. And the sad truth is, not that many modders are investing the time it takes to learn, or are becoming frustrated and giving up altogether.
But it is still too early to give up. Wait until after MotB then see what happens.