Because everything made in the greyzone of "until the IP owner sends us a cease and desist" risks the console producer (sony/Microsoft/Nintendo etc) getting in trouble for allowing the content on their hardware.
In this specific instance the IP owner and the console manufacturer are one and the same. I just don’t see Microsoft spending any effort on porting Fallout 2, though. I could see them remaking the original two though with how popular some of their other remakes have been recently.
Because my man is over here asking for mod for a 26 year old game to be ported to a games console it was never programmed to run on.
It's not exactly smooth sailing to port a complicated game engine to a console, then on top, put a barely finished mod for that game to be put on top of that, and make it run properly.
It may be possible to compile Fallout 2 CE https://github.com/alexbatalov/fallout2-ce for XBOX and install it in developer mode, that would be the best chance.
Don't ask me how, but most likely it will not be an easy project.