1080p, really
Your answer lies with Apple, or the movie studios, or a combination of the two. It most definitely has to do with those in charge thinking in 20th century terms.
Ideally all movies in iTunes would be offered as some are, with four options: rent in SD or HD, buy in SD or HD. Instead, as noted, it might be this or something inexplicably less. Moreover it can change, so worth checking back as that desired might become available. There seems no rhyme or reason, other than in disenfranchising the customer.
Notable also that that all HD fair presently offered in iTunes is only half that, 720p rather 1080p. If which rather good, better than nothing, is no match for the quality of a Blue Ray disc (BD). Some go to great lengths, in time and technical expertise, to digitize their DVD and BD discs. Anyone having experienced the ease and flexibility of downloading media from iTunes and using a digital library might understand why, but in most cases probably neither the inclination or expertise to do as much.
So at the moment one can accept the ease of using such providers as Netflix, Amazon and iTunes, at a certain quality, or opt for the best quality with BD in the traditional form of physical media. At some point someone (with the will to make it happen) is going to realize there really is a demand for the best possible quality, anywhere, anytime . . . at the push of but a button. It will come, all excuses of limitations of broadband and other factors aside. Those thinking otherwise are not firmly in the 21st century, or focusing forward.
Apple could begin by not only offering a new version of ATV 1080p capable, but also offering such media on iTunes . . . as an always standard option.