There are already several responses to your questions, but I do want to add to what my co-mod, troisroyaumes, said. When we, the dark_agenda team, initially talked about the possibility of hosting a dedicated chromatic challenge, one of our main goals was to promote underrepresented sources within all the various chromatic fandoms themselves as well as outside of it.

So when we discussed our scope, one of the things on which we agreed regarding fandom eligibility was that the fandoms would have to be by chromatic creators and that they would also have to be rare. If we had only want to promote all chromatic sources (or all rare chromatic sources), we would have just continued running Chromatic Yuletide Challenge, Chromatic Remix Challenge and any other sub-challenges instead of contemplating the possibility of running our own challenge (in addition to them). (That's not to say that we won't consider expanding our scope for future challenges re: chromatic creator criterion, of course.)