Honestly, there is no particularly good reason. My belief is that because of the way TVos is designed, the available storage space is constantly in flux. Despite there being ample storage for at least a few videos at a time, it's actually nigh on impossible to determine the actual state of available storage.
I know that probably sounds a bit backward but, as were probably all aware, beyond the initial install of an app, additional assets and this is particularly an issue for games. Once downloaded are still marked as potential available space.
So just for example, if you downloaded 20GB worth of apps on a 32GB Apple TV. The logical thinking is that you have, let's say 11GB of free space left (accounting for the OS.) But actually, as far as TVos is concerned, there is still 31GB of available space of it needs it.
Ultimately I think it's down to Apple positioning the Apple TV as a streaming device and they have been moving away from local storage for everything more and more.
And I'm really beginning to ramble now, I'll get me coat