There's AskUbuntu site, but users of other distros have to use Unix & Linux, marking their questions with 'distribution name' tags. How people know when there should be separate site for their disto. How people decide when to separate from the rest?
Area 51 is currently accepting proposals for subjects not already covered by our existing communities. Ask Ubuntu was launched before the broader Linux site, so we did not have a site on that subject. We do not split off individual tags simply to give a topic its own space, so we would not be interested in creating sites for distros already covered under Linux.
When the distribution is sufficiently different from mainstream Linux that users of Unix & Linux are unlikely to be able to help answer questions about it. For example, the only Android Linux questions you're likely to get answers to on U&L are those related to the inner workings of the operating system.