Fox and ABC have always been 720p. The main reason was due to all the sports they carry, and at the time, 720p could be 'smoother' then 1080i. Of course, with most TV's upconverting to 1080p or 4K, it's mostly moot. Also, it can have a slightly lower bitrate without artifacts, so with multicasting, it tends to be more common. I think WTTW went 1080i/720p/1080i/720p over the years.
To make it worse, in areas where Comcast has upgraded HD to MPEG4 (like Chicago), they have been changing most of the HD channels to 720p, even if the source is 1080i. That lets them squish in one more channel per QAM carrier.