According to Wikipedia ATSC standards allow for up to 1080p30. An update in 2008 allows for up to 1080p60 to be used, but since older hardware would be incompatible, it wouldn't be wise for stations to look into it at this point. Must be everybody thought 1080i60 looked better than 1080p30 or there was some other technical reason in the early days of DTV.
It also says Blu-ray discs use 1080p24 and not 1080p60. But movies are still shot on film at 24 frames per second so it makes sense that they chose 1080p24.