Originally Posted by NonMcTubber
And it may also explain why many choose to broadcast in less bandwidth intensive 720i or P.
Actually, 720p and 1080i require a fairly similar amount of bandwidth to transmit with comparable subjective quality, if you factor in both resolution and motion performance.
At the station level, we generally encode for over-the-air transmission in the same format our network uses -- cost and avoiding extra processing and conversion steps are the main reasons, though there are others. At the network level, the format decisions were based largely on the type of programming was perceived to be most important. Back when ABC chose 720p, Monday Night Football was their "premier" live HD program, and using progressive scan was a no-brainer... especially when you factor in slo-mo and overlaid field graphics. Other networks picked 1080i, feeling that resolution was more important than motion for their content.
720i isn't an ATSC standard format... and I've never seen broadcast equipment that offered that as a parameter.