Many channels now seem to be 720p instead of 1080?

with the new hard drive installed (see my previous thread) I noticed that many of the lesser channels had changed names or migrated. SO I did a complete rescan. The channels I’m interested in are all there with 5 green dots, but almost all of them, with the exception of PBS are now in 720p as opposed to 1080i

I’m wondering, either they changed to free up bandwidth for ATSC or there is some other reason. since I guess 720p and 1080i are nearly similar in bandwidth?

anybody more informed on this than I am?