Admiral jimbob said:
Because NWN2 is really a pain in the ass to make mods for.
I was working on a remake of Serpent Isle for the NWN2 engine, and to be honest it's not really a pain in the ass, it just takes a while to get to know and understand the toolset. Hell following vast tutorials that are available and the helpful people on the bio forums I picked it up quite easily.
I think a Codex Mod for NWN2 would be EPIC. I's say start with a good story (tons of choice and consequence, skill checks etc...). Once you have a good solid story to build from, then you can start working on the mod.
I'd be glad to offer my help. I can script, and I can re-skin placables, tilesets etc...