Because Apple wants to sell a lot of hardware. They claim that's what it's all about- selling lots of hardware. Build in 1080p and they sell iTVs to the camp that thinks "720p is good enough" AND the camp that says "1080p or bust".
Gimp it (again!) with only 720p, and you appeal to only one of those camps, which is exactly what we have now. The current

TV is limited to a weak incarnation 720p and the sales are so poor that it quickly went from the fourth leg of the table to "hobby". If Apple wants the new one to continue to be a poor selling "hobby", they hamstring it at 720p- just like a 2006 incarnation. If they want to sell a lot of hardware, they address what is probably the #1 (or at least top 3) identified shortcoming of the current incarnation.
There's always new stuff to build into the next generation. They don't have to leave something like that out so that there's something to sell in version 2 (or is that 3 or 4?). Apple is a pretty smart company. I'm sure they can build it in and then come up with something next year to make the next version more desirable. It's worked for 10 years on iPods, often by just adding a little more storage, making them a little bit thinner, etc.
Myself, this is what I most care about in terms of what will differ it from the

TV I have. If the new one arrives with only the same 720p the current one has, I probably don't buy it. On the other hand, if iTV shows up as exactly the same box, software, etc that we have now- only with 1080p chipset inside- at even the same price it's offered now... I buy it day 1.