They eventually encounter issues with the OS when working with the game and each team decides that "Enough is enough", and so ends vista support. Or something along those lines. Vista is .46% of the PC market (and no doubt less among gamers), so literally the moment they encounter something that's vista's fault there's no reason to fix it.
Pretty much. It's either supporting the legacy OS or making the game worse for everyone else using a modern OS. (The third option of supporting both is comparatively financially nonviable given the resources can be put into doing something actually interesting for the game instead for the 99.x% of the users using a modern OS.)