Cost. First off the minimum size for any respectable HD panel nowadays is 32". 42" is the sweet spot in my humble opinion. If Apple were to make an actual TV they would have to market at a very high price indeed to make the same margins as their other products, somewhere even above the Mac Pro price. Pricey TVs don't sell even if it's Apple branded. And they take up a lot of space, weight a lot, therefore costing more all around to store, ship, display, etc.
Distinguishability. Go to your local electronics retailer and look at the tv offerings. Other than branding and tv sizes, and sticker prices, what else jumps out at you as different at one glance? How can Apple differentiate their offering in the face of that reality?
It's also mentioned over and over by analysts and observers that despite it being a $100 plus billion industry it is very cutthroat, with paper thin margins. People hold onto their TV sets for 5 plus years at least if not longer. It's hard not to see why getting into this industry seems foolish. Consumer electronics = cutthroat.
Then again this is Apple we're talking about. See iPod, iPhone, iPad. Not a bad track record considering.
In comes Siri on iOS.
Apple releases one tv, a 42" HDTV powered by the latest ARM and graphics architecture, running on iOS, controlled by iPhone, iPad, and most importantly, Siri voice controls. You've seen what Siri can do, imagine what it will be capable of. I for one would love to do away with the tv remote, bloated with funtions most of which I will never use. "Siri switch to 'insert tv channel here'.""Siri, record show","Siri, pull up news ticker/weather/stocks","Siri, check email"
By being iOS powered it also leverages the app ecosystem Apple has built around its other iOS devices and tie them in further. Current iOS users would be heavily inclined to buy Apple's TV rather than consider other offerings. Should I get a new tv or computer? How about a tv that has a computer in it?
It also becomes a formidable gaming machine. Xbox360/PS3/WiiU beware. Not only will this take on the TV set industry it'll take a bite out of the console gaming industry at the same time. Win.
Siri will distinguish an Apple tv from the pack in terms of usability, functionality, and ecosystem.
As for cost, the flat panel is what will make it cost more. The possible income from multiple industries - tv, gaming, computing, movies and tv, should more than make up for any thin margins on the tv itself.