I'm quite sure I've seen at least Warhammer and Cthulhu games on Roll20 with implemented skills and challenges resolved, so I guess there is more to that.
Besides it's in best interest of any VTT to have more options than just D&D.
It's the same issue though. VTTs greatly constrict the range of games that can make an impact, due to their level of support.
And it's a vicious circle because only games that have a large enough group of players will get significant support.
I playing Traveller on Foundry and it's being developed by volunteers. Rules implementation is very spotty. The degree to which a game is supported will straight up decide what system I run with; I don't want to deal with poorly coded frustration. And this leads to a prevalence of rules simple games. Numbers + monetization will be the cure, but some systems will never get developed due to complexity/obscurity.