Because it doesn't know how to do that. Dish only has VOD in 1080P and if you want to order it in 1080P the 722 will look to see if that is possible and if it is not it will not let you order it.
Are you talking about the setting in the menus (menu-6-8)? In the HDTV setup, the receiver will only go to 1080i, even though there are 1080p VOD movies available.
Because, with the exception of VOD, there is no 1080p that is broadcast from networks or cable channels. Mostly all 1080i with the exception of ABC/ESPN and Fox (720p).