There’s a number of reasons. Here’s a couple.
First of all is time. This Community Hub is only a year old. Most of the other Ubuntu derivatives long ago setup their own ways to communicate between the developers and among the users. We’d of course be delighted if any of the flavours wanted a category or more here, but I’m not about to push them to do that, because they know their teams and communities better.
Second there’s branding. This site is very much “Ubuntu” branded (see the giant orange bar). Each flavour has its own identity, colour, theme. It could very well confuse people and dilute the flavours brands.