The same reasons 10s of millions have bought Skyrim, GTA 5 etc. on console? Cause most people don't have and don't want a gaming PC?
For what it's worth (unless it's changed?) you can do mods on Xbox. I'm pretty certain you can do mods for fallout, elderscrolls etc. but you have a smaller pool as they have to be certified (or something). Doesn't have the same options as modding on PC but it's not a bad compromise considering historically it was always a PC only thing.
Ok you took “I don’t know why anyone” a bit too literally there.
I personally have no interest in playing it on a console.