Backward-compatible Xbox 360 games will still be available for purchase on the Xbox One and Xbox Series X|S stores, Microsoft says.
So no, presumably Microsoft just doesn’t want to deal with the tangle of close to 20-year-old code that holds up the Xbox 360’s store interfaces.
Uncompressed textures and uncompressed audio for all languages at once (this started with the 8th gen consoles because their shitty CPUs couldn’t handle real-time decompression), so a lot of space is being taken up by audio that’s never used in languages you don’t understand because at some point in the last 20 years the gaming industry forgot how to create checkbox installers.