Could it be that network effects are much stronger for Minecraft? Morrowind is a single-player game. OpenTTD has network play but I have always played it solo and I suspect many others are the same. I don't play Minecraft but from what I have seen, cooperative play is a big part of it. Players may be less motivated to work on a FOSS alternative when they know that they would then also have to convince the community to move to it, which is much harder. Same way it's difficult to build a real FOSS alternative to WhatsApp, for example.