I think a large part of that may simply be LW'ers being more familiar with UFAI and therefore knowing more details that make it seem like a credible threat / availability heuristic. So for example I would expect e.g. Eliezer's estimate of the gap between the two to be less than the LW average. (Edit: Actually, I don't mean that his estimate of the gap would be lower, but something more like it would seem like less of a non-question to him and he would take nanotech a lot more seriously, even if he did still come down firmly on the side of UFAI being a bigger concern.)