I don't know if it has been mentioned but at QuakeCon 2011 Carmack stated that the textures had been compressed to optimise for 720p - the result being that on higher resolution displays, like the multi-megapixel resolutions available on PC - would suffer. He talked about wanting to do a dual-layer Blu-ray release for PS3 but not wanting to take the risk with the release approaching, and warned that the 256MB RAM of the PS3 would likely have been an issue. He also mentioned that he wanted to do a DLC release with a level that has the full textures.
So basically, the textures are crippled due to design concessions made for consoles. It's not because systems can't run them - it's because they couldn't spare the disk space (something that isn't an issue with digital distribution).