Because this OS is largely built on the current iOS 9. They did the same with the previous Apple TV OS which followed the normal iOS version numbers fairly closely. Makes it far less confusing for everyone when you say "Make sure all your devices are running OS 10.3" rather than "Make sure you're running tvOS 1.4 or later, iOS 9.4 or later, and OS X 10.11 or later."
Except the released versions for tvOS (9.0) and iOS (9.1) are already out of sync. And there's no prayer or intent of aligning OSX versions with iOS versions. So you are always going to have to deal with three versions. Personally, I find it very confusing to try to sync with two similar-but-different release versions.