Primary because most cable subscribers get local channels via cable and don't have an antenna to get local channels. Setting up an antenna can be a hassle, and likely make switching much less likely. Given much of Apple's target market is probably cable users local TV becomes a must have.
I suppose that is true. I guess it can be a hassle, though a small one. For 80% of the US population it is about as much of a hassle as getting a new table lamp: Take it out of the box, put it anywhere, plug one wire into your TV, and plug the other wire into the electrical socket (for the amplifier). Today's omni-directional antennas have gotten really good, and people within 60 miles of a tower can get a great signal without worrying about placement. Amplifiers have also gotten tiny, and many work off USB power which many TVs have now. The folks that need to bother with rooftop antennas, directional antennas, etc. live in a rural area far from a city (and make up less than 20% of the US population).
Also, totally a dream and very unlikely to happen, but it would be great if Apple bought Tablo and incorporated their hardware into the new AppleTV, or as a companion box to the AppleTV (call it AppleLocal?). Either way, as soon as Tablo has an app available on AppleTV, I am buying it without delay.