But the answer isn't:
"Because we aren't paid, and only a few of us care to spend our free time supporting hundreds of users each day in exchange for nothing more than personal gratification."
I'm sorry, I didn't realize that this was a test.
I see you're assuming that you know us better than we know ourselves, which is really not the proper way to start a survey on social participation in a community. You must learn to accept answers, even if they don't fit your ideal conclusion. You must observe from an outsider's perspective with a willingness to learn and to adapt your theories.
Because 90% of the fora in the world are community driven; all these people do all that helping, answering and discussing for free. . . And in comparison with many other fora I found this one was slow.
Again, you're comparing apples to oranges. The WordPress support forum cannot be compared to "90% of the fora in the world" because it deals with support for the WordPress blogging platform and not whatever "90% of the fora in the world" cover.
This forum, like any other, is powered by humans with their own individually, including skill level and motivation. Human individuality is a variable which cannot be accounted for in a mathematical or scientific process when conducting a statistical survey.
Is that because maybe people don't like the forum software being used? Is it hard to get started?
No, it's really because we're humans with our own individually, including skill level and motivation. There's really no other answer.
But what is it then that keeps all these happy users of WordPress from engaging in the forum as well?
Those who want to engage in the forums do, and those who don't want to engage in the forums simply don't.