Mini DisplayPort would suffice though, as Thunderbolt is meant for more than just video display and as Thunderbolt requires an extra chip, the price of the Apple TV would go up.
To learn more about Thunderbolt (TB) and its compatibility with Mini DisplayPort (MDP):
And since the Apple TV is meant as media consumption device for moving images like films and TV series and all that other stuff, TVs are the predominant recipients of signals from an Apple TV, and since TVs don't use Thunderbolt or even DisplayPort or Mini DisplayPort, why include such port in the first place, when almost all TVs come with several HDMI inputs anyway?
Consumers using a computer display as a TV replacement are not the target audience of the Apple TV.