These aren't global numbers, but this is the breakdown for sales of interchangeable lens cameras in Japan in 2012:
Canon 27%
Nikon 25%
Olympus 14%
Sony 13%
Panasonic 11%
I don't think smaller brands are being pushed away from this site, though. We have a larger-than-market-share representation of Pentax users among the active site users, for example (Pentax being at something like 3% in the above).
So, that's a lot of it. It's also somewhat down to proliferation of models — Nikon and Canon have larger lineups, and tend to refresh frequently (especially at the lower end.) So that's another big chunk of it.
And finally, a lot of those Canon and Nikon model-specific tags are kind of gratuitous and not really helpful on the questions tagged with them, like http://photo.stackexchange.com/questions/35466/imported-photos-have-much-better-saturation-before-lightroom-completes-loadinghttps://photo.stackexchange.com/questions/35466/imported-photos-have-much-better-saturation-before-lightroom-completes-loading or Will the 18-55 and 55-250mm kit zoom lenses cover my requirements in Europe?Will the 18-55 and 55-250mm kit zoom lenses cover my requirements in Europe?. Or Will keeping the lens on-camera help protect from dust and fungus?Will keeping the lens on-camera help protect from dust and fungus?.
One can make more or less charitable explanations for that, but a middle-of-the-road explanation might be that since these cameras are more popular in the mass market, there tend to be more questions from newer users who aren't really sure if the specific camera model really matter to the question. On the other side of the same coin, users with less common models may quickly learn that making the question overly-narrow is less likely to get results.