this might be due to funding. economically, it makes sense. lucasarts probably hasnt given them additional funding once lucasarts got what it wanted finished (the us xbox version), and thus little to no funding for finishing up work on the pc version.
in contrast, obsidian is probably being paid good coin by atari to be working steadily on nwn2, thus the logically thing to move the majority of the staff over to that project since its actually paying.
im not saying thats a good thing, or that im not upset about the poor review about bugs for the pc version, but that in an economic sense, thats probably the reason.