I'd gather that if there were a lot more religious people posting on Less Wrong, there might have been a similar injunction about discussing religion. More religious people might have resulted in more threads devolving into atheism vs religion debates (not really "debates" but flame wars) which would detract from the goal of the blog which is about improving rationality.

It probably doesn't help that a lot of the initial posts on Less Wrong -- meaning the Sequences -- are implicitly (and sometimes explicitly) anti-religious. Which probably functioned to dissuade more religious people from joining the community initially, since religion was seen as a go-to example of irrationality.