One difference between MO and wikipedia is that here you get very quickly answers or remarks on what you have written, and it is then very much more attractive. Going to wikipedia for finding something for oneself, you directly have the answer you were looking for (or dont find it), but really writing new stuff takes times and you don't really obtain a feedback.

I do not say this is a good reason, I just say that it is a reason why people go more often to MO than edit Wikipedia's pages.