My comments expanded enough to be some kind of answer.
I think there are a number of reasons why the rate might be different - some have been discussed before. On SO or math.SE (which I participate a little on, mostly on R and stats questions respectively), questions tend to be much more specific and focused, with direct, short answers. ('How do I do this?' ... may often answered with 'call this function' or with a couple of lines of algebra.) - those are not the norm here; often the asker doesn't even know what they want, let alone how to express it.
In particular, I think a lot of questions on SO or math.SE could be answered by a competent undergraduate or enthusiastic amateur - and there seem to be plenty of both around those places.
Apart from homework-like questions, there aren't so many of either here. Many questions are subtle and the breadth of knowledge required to tackle more than a tiny subset of them takes a long time to accumulate. I think I have answers in more tags than anyone else now (about 450 I think), yet I can really only competently answer a very small fraction of the questions we get.
In the course of answering questions over a few days, I often find myself doing algebra I've never quite attempted before, running simulations I've never run before (and writing and debugging code to do them!), suggesting novel or tweaked test statistics and exploring their properties, comparing the properties of several approaches to a problem, coming up with slightly novel way to visualize some data, reading papers to follow the history of some little technique, reading more papers to even figure out what a person is asking about ... and so on. That is, a lot of questions here take actual research effort. Sometimes hours of it.
We have a pretty dedicated but small group of people who answer a lot of questions and an even smaller dedicated group who put effort into clarifying questions and getting posters to improve/focus their questions enough to be answerable (anyone who is reading this is likely to be contributing in both ways). Those need to expand.
We do quite well on some things - all our review queues are empty, almost all the time. SO would love to have that statistic. Nothing stays in a review queue for long. The survival time in the suggested edit queue often seems to be a couple of minutes at best.
I am not sure how to get more people answering questions here. I sometimes do a bit of pointing to good answers here while I am in other forums, and that sometimes brings a few people in. I do talk to people about the site, but I seem to be singularly unsuccessful in convincing other people to actually come and participate.