I won't claim this explains the entire disparity, but starting out on Robin Hanson's blog has to have had a pretty strong effect, given the nature of RH's frequent writings on gender politics. Every LW veteran from OB is someone who was able to agree with, tolerate or ignore those posts; and that's going to be a very slanted demographic.

(I won't go into this issue any further to avoid massive mindkilling, except to note that I think RH applies the wrong kind of cynicism to human beings in general, and that I fall into the "ignore" category when it comes to those posts.)