You can get champion mastery in URF, but not on Treeline.

They all require different algorithms. Most likely they focused on the Rotating Game Modes so they could be pushed out, instead of delaying them for a mode that is fine for now without.

Yeah joke modes get more attention than Treeline, it's sad really. It is listed under the classic queue in client, it should grant champion mastery. Of course, they might just be trying to kill it off via negligence that way they can shaft it like they did Dominion. The funniest part is that the HEXAKILL: TT will give champion mastery, but the actual version will not. 10/10