Having given this topic a tiny bit more thought, and looking at my posting patterns, I've got to say that it's simply because discussion here is too focused. It's either tech support in the computing forums (with a bias toward hard disk related issues), or debates in the B&G.
I've been actively posting at another forum since September of last year, and I've already racked up 900 posts over there, while I'm not even half of that in almost 3 years of posting here. The differences between this forum and the other are many:
1) More active. This one is a no-brainer; there are many more active people posting over there, so there are inevitably more posts that interest me.
2) More character. The SR Computing forum has always struck me as being somewhat sterile. Sure, you'll have an occasional troll, and there are plenty of disagreements, but there's something I can't fully describe about this place that's different. It's like most of the posts are written by anonymous people. It's hard to get any sense of what people are like (or at least what their internet persona is like), so there are no real relationships formed.
3) News is more active. This is something that really is beyond anyone's control, but the news portion of the other forum has new items on a daily, or at least near daily basis. This is another obvious one; more news = more topics of discussion.
4) Posts are typically shorter. While there are many more posts, at times the other forum resembles an IRC chatlog more than a forum. I'm not sure if this is a good thing or not, but it sure leads to more activity. I don't have a particularly short attention span, but shorter posts are appreciated by most people.
There are plenty of other differences, but I think these are the key reasons why I spend more time over there than anywhere else, and also why I visit SR about once or twice a day, a small fraction of what it used to be.
Clearly, real life is another issue, but I have just as much time to post as I used to, so it can't just be that.