Becuae I find linear RPGs quite annoying and much prefer sandbox. I don't mind them making other games but I don't want them making games in my favourite CRPG setting.
Well, I don't think a non-linear sandbox Elder Scrolls RPG made by Bethesda could be called a spin-off, it will be a full TES game
I also prefer a sandbox RPG and I prefer a Bethesda made TES game, but if a more linear adventure TES game would be made, like Red Guard and Battlespire I wouldn't mind if it was made by Bioware, as long as the main TES games are made by Bethesda and are sandbox RPGs.