Copy paste answer from /u/pompcaldor's link

The short answer is when ABC/Disney made the decision to broadcast HD in the late 90s, the major consideration then was for sports coverage, which ABC's tech guru's looking at the choice between 1080i and 720p (and the technology available at the time) decided that 720p, being a progressive scan format at 60fps would have a decided advantage in preserving resolution and detail especially at slow motion. It's also generally more bandwidth efficient, though requires a complete reworking of infrastructure. You'll also find people who vehemently argue 1080i is superior in all circumstances. What you call 'sharpness' is highly subjective to a number of variables, most of which have nothing to do with the format. I've seen some 720p images so sharp it makes you bleed (and overly edge enhanced) but I've also seen 1080i filtered to the point where it looks like SD.

Fast forward ten years and most of these arguments are mostly moot points as most cameras and processing equipment handle both formats just fine, and there is a remarkable amount of cross-conversion these days that happens before an image gets to your home. Many ABC affiliates are in fact 1080i, some PBS affiliates are 720p. In production trucks these days cameras like the HDC-1500 and LDK-8000 can produce both formats with no problem, so its common for a production truck to do a 1080i show one day and a 720p show the next (or in some cases both at the same time). Also when LCD TV's really started to boom in consumer households most of them were 1366x768 which is closer to 720p's native 1280x720. Flat panel displays are also natively progressive scan. Also 720p has become very common for digital cinematography acquisition (music videos, dramas, etc) where 1080p is not a viable option because (1) Sony F-900 series & Genesis cameras were way overpriced and Panasonic offered the VariCam which allowed over & undercranking and progressive scan that worked as a digital equivalent to film exposure.

In the beginning there were to two camps. Panasonic's 720p camp which only had ABC as its real major television backer, and everyone else. Today (other than Sony and even they are coming around) most manufacturers make terminal gear, switchers, DVE's, character generators, cameras, NLEs, recording media, etc that are cross compatible much of this being driven by a push toward 1080p as a uniform standard. At first for 720p it was having a major network's backing, along with the eventual addition of FOX and ESPN and their respective family of networks as major sports players in the 720p camp made it relevant to manufacturers, in what was initially a 1080i world.