Bandwidth. Easy answer, it's like watching an early YouTube video (now some are nicer but they all used to be very grainy/blurry) vs a DVD, they might both have the same 480 lines of resolution, but the DVD had more bandwidth.
Longer: Every digital OTA channel has 19.2Mb/s total bandwidth to be broadcasting using MPEG2 compression. (I think 1080p Blu-Ray uses 54 Mb/s, 480P DVD used about 11Mb/s) That can either be used for one fairly nice 1080i/720p HDTV channel or some combination of "HD" and/or SD channel(s)
Adding subchannels takes away bandwidth from the main HDTV feed, so generally subchannels get minimum bandwidth to help out the HDTV feed. I've heard that the only acceptable HD/subchannel solution is 720p main and 480i sub. (like 19-1 ABC, 19-2 CW) because 720p can get by with less than the 19.2MB/s better than 1080i
WTVP is near hopeless running 47-1 1080i, 47-2 480i, and 47-3 480i even the main feed suffers from a lot of pixelation when any subject is moving, and the 480i sub channels are not setup with enough bandwidth for a fast motion sports event. Plus MPEG2 is not perfectly ideal for interlaced resolution vs progressive...