Most US cable markets are legal monopolies. My city is. For cable, there's only ONE provider, so they don't really care if they sell me a crappy set-top box or not. They're still getting theirs.
@Minh: It would allow for the cable companies to subsidize the hardware. Instead of renting you a standard set-up box, you could have an ATT U-verse xbox or a Verizon/Comcast/Whatever xbox. WIth each box customized to work on each network. It makes a lot of sense, considering how much of the Xbox is used for media consumption.
That said, the local phone company are providing internet and TV now, too. But they are consistently a step behind in terms of speed.
I know Xboxes from multiple OEMs would be a nightmare for devs. They have to test their games w/ all the different component configurations. Hello PCs.