For those with Smithsonian Channel (now only 720P), or those who may have lost the channel, there is an alternative for those of us with smart tv's, or other devices to get apps on the Internet.
If you have access to "You Tube" you can search for "Smithsonian Channel". The channel now shows the most current episodes in Blu-Ray quality (1080p) just a few days after they were shown on Comcast linear cable as new, but only in 720p (1080i before the down-grade).
The same can be found for NASA channel (also in 4K sometimes). National Geographic, Nat. Geo, PBS, etc. There really isn't any technical reason for Comcast to not supply these shows in 1080p, if they allowed for the extra broadband to have channel in 1080p verus 720 p. They do have cable-boxes capable of 1080p output.
Comcast is using 720p because they can cram more channels into each QAM frequency. Before, they could only get 4 channels, and even that had some artifacts. By using 720p MPEG4, and agressive compression techniques that confuse non-X1 boxes, they can get 10 channels into the same space.