Angry Grimace
Two cannibals are eating a clown. One turns to the other and says "does something taste funny to you?"
I don't think there's any inherent principle at work here. They're not gonna print, say, Zendikar fetchlands because they're an entire cycle that can sell a future set on their own, but Horizon Canopy is a weird one-off and is unprintable in most Standard sets due to the weird concept and specific card design.
I mean, I get what you're saying but haven't printed Grove or Canopy (both of which see heavy play) in any of the 4 Masters sets yet. I wouldn't be surprised at all if they think they're somehow required to print a full cycle and not just one random land.