They did. The regulators, and the market, appears to have settled on 1080i60 (1080 lines of resolution, interlaced, 60 hz - 60 half-frames per second) as the main standard for broadcast. The main reason for using an interlaced broadcast format was due to the limited internal bandwidth of analog tube-based televisions. They could have just as easily settled on 1080p30 (1080 lines, 30 full frames per second) which would have no higher broadcast bandwidth requirements, and would be easier to deal with on the increasingly common fixed-pixel displays (Plasma, LCD, LCD and DLP rear projection, etc). Tubes are basically a dead technology, which although good, will never be seen in any bigger sizes than currently available (34" diagonal for a 16:9 widescreen set). Anyway, it's not the end of the world, but it's not ideal, either.
Given the amount of logic required to uncompress digital video, how hard would have been to allow any resolution up to 2048, and any frame rate up to 60 (or even 120) and let television sets deal with it as they see fit? Would it cost significantly more to produce a receiver that could convert a 2400x1800@72 image down to NTSC than to make that was restricted to 1080i resolution?
It seems to me that for viewing movies, 24fps is better than 30fps (and raster-scan sets could display it as either 72 frames/sec progressive, 24 frames/sec with 3:1 interlacing, or 48 frames/sec with 2:1 interlacing). Was/is there any reason not to allow broadcast in whatever format best describes the source data?