Hexakill

Probably was put on Twisted Tree Lines because maintaining 6 players on the Rift is harder/more work than on the Twisted Tree Lines.

They made that map, better use it for something

It's been alternating every year (prior to the rotating game modes). One year it's Twisted Treeline, the next year it's Summoner's Rift. I would say that Hexakill originally was played in Twisted Treeline back 2014, I think. But I can't confirm.