Probably because people don't receive beyond 720p in most areas (I know I don't) through digital cable or satellite. However, I receive 1080p by streaming through my xbox, so Apple TV won't even have a hope of being a replacement.
It is probably rude, but I'm still too curious and have to ask: what part of the world are you living in, where you don't get 1080p satellite coverage?
Canada! Say hello to the bullshit of a government supported monopoly that is Bell. Even the other satellite companies don't provide 1080p, IIRC there is one within Canada, but most people I know who are receiving 1080p are catching US signals to get it (better selection anyway I guess).
Our government prevents outside companies from entering the markets and make it damn-near impossible for internal companies from entering the market too to ensure Bell, Rogers and Telus keep their monopolies. Telus barely has a TV service. Rogers has cable, but are locked out of many areas, thankfully my area I get Cogeco which provide relatively cheap, very good quality internet.
I don't watch live-TV for shows I need in 1080p anyway, so I deal anyway. I don't exactly want adverts when watching a SG:U in 1080p; kills the experience.