I rarely see any questions regarding Kubuntu, Xubuntu and so on. Does it mean that these operating systems are also rarely used as compared to Ubuntu, resulting in less questions on the main site?
IMO it is just that most questions are independent of the desktop, they are about specific applications or the underlying OS, rather than the desktop, so they appear to be about Unity Ubuntu but the OP may have a different distribution.
Looking through the tags:
Xubuntu has 3081 questions and XFCE 1085
Kubuntu has 2544 questions and KDE has 1422
Ubuntu Gnome has questions 346 and Gnome has 5496
Lubuntu has 2789 questions and LXDE has 355
Ubuntu Mate has 183 questions and mate has 380
This is a rough estimate as there are many more desktop related tags but there are plenty of questions about different versions.
Ubuntu is, of course, the most popular because it is the "brand name" edition and where new users go first but I would not say they are rarely used, just it tend to be more experienced users who try other distributions and they also tend to ask a lot less questions.
I'm not currently a member of the Xubuntu leadership but brought up the issue of utilizing AskUbuntu during that previous period. I'm still trying to sort through mis-tagging of questions that drag them under the Xubuntu tag when they don't actually belong there. That's on my own, though.
Xubuntu's leadership generally doesn't respond to questions on AskUbuntu due to a lack of human capital to spend on it. There is only so much time to go around. My own involvement here involves me disappearing for periods as I have other things I have to be doing, too.