This is because those channels are still MPEG2 and weren’t converted. The thing they did which reduced quality so much wasn’t going from 1080i to 720p, but lowering the bitrate and quality of the video so it takes up less bandwidth (while going to MPEG4). All the networks are probably still MPEG2, as well as a few other channels like ESPN in certain areas. MPEG4 also has better compression than MPEG2, so the converted channels should in theory have smaller streams for the same quality. However, Comcast went well beyond that and further reduced the bitrate, reducing quality from what it was before.