It's not that complicated. WPCentral was basically first and best when it came to Windows Phone news and content back in the day. Because they were early and very active they became the defacto authority on the topic. They are to WP blogs as the iPhone circa 2009 was to smartphones.
Crackberry too was one of the sites that basically created the brand-focused mobile phone blog/community and it at one time was maybe the undisputed authority on BB news.
When it comes to iMore and AndroidCentral there's two things at play. First, there's a ton more competition for eyeballs in those spheres. There's a long list of Android and iPhone focused content creators and Mobile Nations can't claim to be unique or clearly better than everyone else. Hence, those audiences are far more fragmented and while they might read iMore/AC semi-regularly they don't feel they have a stake in the community since it's just one of several stops. For WP and BB, these are the only sites we tend to visit and thus we're really attached to them.
A second factor is that iOS and Android get a TON of coverage on the big-boy general tech news sites like BGR, Engadget and Ars. Because of the high-profile of those sites and the depth of their coverage on those two platforms, there's much less need for readers to seek out a brand-focused site like this one to get their fill of news. iOS and Android news is everywhere, all the time.