More access to tech and free time especially for the ethnic majority who unlike the racialised underclass are not subject to greater degrees of poverty that curtails those things, is in (white majority)western countries is one factor.
As well as the majority of such sites being English language(which is the majority language or the primary second language in most white majority countries). e.g. A Spanish or Arabic language forum would have significantly different demographics.

And outside of Hexbear specifically, PoCs often get banned by white admins/mods for bringing up racial disparity and their experiences with it, under the guise of removing 'divisive politics'