Everyone leaves for different reasons. Not everyone has a need to dissect what they used to believe in detail, some no longer have even family and friends tied to the organization and thus no interest in the topic in general.
You also have to remember the vast majority of the general population doesn't visit forum communities on a regular basis, so it should not be surprising that a lot of ex-jws don't. And that is before you figure in the fact that a lot of them are not tech savvy due to having been in the organization so long.
A lot of kids that simply don't believe in it or want to do their own thing who are leaving are not going to care about a site that is discussing the JWs, they left because they don't care about it anymore.
That being said I know there are a lot of lurkers and people using the site as an information source as well. And the community is still a good size in my opinion. So no I would not say I am surprised and I wouldn't really say there are so few people on this board either.