It's pretty simple, the infrastructure isn't ready for 1080p. Very few are going to wait an hour or more to download or begin playback on a 1080p movie. For the vast majority of users 720p is about all that their internet connections can handle. Besides that, it seems like the content providers are mainly interested in protecting their current cable, DVD, and Blu-ray distribution models and thus they (the content providers) really don't want internet-based delivery to succeed (at least not with any type of cost/quality parity -- i.e. iTunes movie rentals and TV show purchases are just too expensive right now).
Given the above, why should Apple rush to update the Apple TV hardware? The factors that will make or break the Apple TV are for the most part completely out of Apple's hands (internet infrastructure/bandwidth and the relatively high cost of the content). Note that Netflix's CEO says that it could be another three years before internet-based streaming becomes a majority delivery mechanism over physical discs (for Netflix, and Netflix is rumored to be the largest streaming provider in the business).
About the only reason I could see for Apple to update the Apple TV hardware is if a new design could increase Apple's margins (or if a new design could be sold for less while holding margins about the same -- possibly resulting in more unit sales and thus higher total profits). We might be getting close to that point, but I'm not expecting Apple to introduce a new design with radically different capabilities. In any case, 1080p decode might be possible without increasing costs or lowering margins, but I really don't expect Apple to offer content in that format so new hardware alone can't make the iTunes Store experience any better than what we have today.
Generally, I can agree with these comments, though I'll differ a bit on the last one.
I see it a bit more harshly than presented here. These video-side producers fully or partially own music-side companies or are closely affiliated with the music side. They know very clearly what happens when you allow Apple to get too much hold over music, and they don't want the same thing to happen to
their content. People will tout that Apple iTunes
saved the music industry, but that industry would like nothing better than to get out from under Apple's tightly controlling thumb. These video-side players don't want the same fate.
The broadband capacity argument is a good one, though I offer that the owners of the pipes will not expand their pipes until they have to do so. Either they feel pressure of competition (which is unlikely since so many of us have 1- maybe 2- choices for broadband internet, (both) priced about the same for about the same speeds; as service monopolies- or pseudo monopolies, the established Goliaths will gobble up or crush any new competitors), OR they must feel the pain of revenue loss (people quitting their service in sufficient numbers that they have to do something; this is unlikely when they are the only (or 1 of 2) broadband game in town). To this point, I've made the case in similar threads that Apple should
LEAD this charge, since they only have control over what they build. If there isn't greater demand for more bandwidth, there is little chance that such bandwidth will be created (as an example, look to the greater broadband created then filled with telephone VOIP service, or by the bandwidth freed up by converting analog television to digital, then filling that space with services like VOD; thus, even opportunities to expand bandwidth within existing pipes doesn't result in such bandwidth getting passed on to broadband service. Or, another way to say this is that additional bandwidth appears to be readily available, but it is allocated (in some cases you can see it in BUSINESS broadband packages offered through the same pipes... at higher prices of course) away to other revenue-generating services). So putting more demand on the system (with Apple making this move- the one Apple has total control over) seems like it would have to come BEFORE these related players play ball.
Similarly, there is NO reason whatsoever for any Studio to test the market demand for 1080p video via iTunes for

TV until there are lots of

TVs in homes that can play 1080p video. It is IMPOSSIBLE for them to even find out if there is a big market for 1080p video for

TV as long as there is no such

TV hardware on which to play it. It's not hard to imagine video-industry executives even wanting to test this likely cheaper distribution medium (as opposed to printing discs & packaging and giving a big cut to Walmart, distributors, shippers, etc). But they can't even find out if this is a profitable alternative until there are lots of 1080p

TVs in homes.
It is for these kinds of reasons that this "chicken & egg" situation probably must be resolved by Apple making the move to 1080p hardware. And by promoting this hobby to a more mainstream focus, they can push a 1080p

TV into many more living rooms. With entrenchment of each additional unit, the lure for potential profit will tempt every Studio to consider testing 1080p iTunes content. With success, the iTunes Store could quickly deliver an enhanced experience.
We don't have millions of songs available in iTunes because 6 Million iPod units have been sold. It was selling
tens of millions of iPods that enticed every Studio to want to have their product in iTunes, and almost every musician to find a way to get their songs there too. The Studios don't want DRM-Free music, but they had to go there (too) because that's what the market demanded. Since that market is mostly iPod owners, they had to play ball with Apple.
Similarly, if Apple would build the

TV that the mass market wants- not one that is 3+ years old that can just barely play back a minimal version of HD video- and then push it as an iPod for your TV, with similar levels of entrenchment comes (video-side) Studio support. With numbers, the lure of potential profits will motivate at least one Studio to test 1080p content, and success (profit) will motivate the others to (quickly) follow. Stick with a 720p

TV platform and it is IMPOSSIBLE to even test the profitability of 1080p content on

TV. Thus waiting for the content to come BEFORE the hardware is putting the cart before the horse.
Hobby or not- Apple should LEAD the way. It is the ONLY way to get the other players to step up and deliver their parts. Waiting for those others to move first means almost never getting there.