I don't know why others don't follow with this. I'm sure there are reasons I can't see, but it just seems easier with a rolling release like Arch or Tumbleweed.
They can still snapshot at a point in time to create the LTS release. Maybe it's more work on their part since with a rolling release there can be so many packages that change rather quickly?
I know it's almost enough of a pain to do the upgrade from release to release sometimes to make me consider just wiping and starting over (I usually do that with my laptop and Fedora since home is on a separate drive).