I would have also guessed its the version of software your set is running. Even if it is the most recent version for your set, it may not be the most recent version that the apple tv app is compatible with. That is often an issue that built in tv streaming software runs into in a few years time.I too am expecting it to be tied to the App, not hardware.
I just went to talk to my friends that run the High end TV store ... asked about why my TV wouldn't get Apple tv ...
We tried on thier TV's and it showed up each time.
He said it much have something to do with the version of Google OS on my TV ... (It is up to date, most recent for my set).
They said that the update IF thats what it is, would come from Apple.