The top reason is, the mass market loves integration. Using component audio systems as an example, while the early adopters and audiophiles loved component systems, the mass market purchased all-in-one solutions, and even audiophiles migrated from separate pre-amp, tuner, and power amp to integrated receivers.
I'm hoping for full integration of cable TV and Apple TV services (no multiple remotes, everything controlled by a sleek Apple Remote and iOS devices). Everything you need is connected to the same device. It's probably too much to hope for a TV that includes the cable box, but you never know.
Another feature ripe for integration - gaming. Kicking the offerings (and revenues) at the App Store up several notches. Think Apple's going to let X-Box, which is moving in the direction of Apple TV, go unchallenged?
Integration brings synergies. Today's TVs ought to have FaceTime/Skype capabilities that do not depend on outboard iOS devices and AirPlay. That means cameras and mics. And from that? All the uses iOS devices currently make of those sensors, plus living room-inspired uses.
Of course, I'm presuming the TV will have full iOS capabilities, and new additions to the API. I can't see how Apple wouldn't do that. They need to bring first-time Apple customers into the ecosystem. Third-party software adds value and an added revenue stream. Have they been intentionally holding back on that with the current Apple TV product? Perhaps. Let the App Store developers loose on this!
Deeper integration with the Apple ecosystem means FaceTime, iMessage, Contacts, Calendars, Reminders, Passbook, etc. (via iCloud, of course). While AirPlay is cool, a truly integrated TV would reduce the need for outboard input. The TV might be a family's first step into the Apple ecosystem (just as iOS devices have been for many Windows users). Smart thermostats and lightbulbs (and, I'm sure, security sensors, remote cameras...) belong in every home. I don't think Apple's going to ignore the opportunity.
Apple didn't build the first smart phone, but when they did... So, we can look at what's already out there in "smart" TVs, and expect that Apple will not bring out a me-too product.
And for those who aren't ready to retire their old HD sets, I'd expect an outboard Apple TV product with greater capabilities, too.