Mill has been a part of the Generalist Cube from the very beginning. Back when it was first constructed, mill wasn't very good in constructed Magic, but in cube, it had a new life. Some iconic cards and high impact cards (as well as a few bad cards that seemed like they might have had potential) found their way in.

It wasn't ever really anything other than fringe, but it was playable and wasn't oppressive. Even when the cards got better (Mind Sculpt flipping out for Startled Awake and the like), the deck remained as mostly just a fringe strategy.

A big part of that is that the typical play environment for the cube was multiplayer, while the single player games tended to have at least one player running the 60-card special. With those constraints, mill cards needed to be super high impact or they did near nothing.

However, time has passed, and these days the newer group of players doesn't tend to like the 60-card special, sticking to the more consistent 40-card deck. In turn, mill has gotten a fair bit stronger, with cards like Increasing Confusion winning much sooner than they used to. Because of that (and a few other reasons), big mill has slowly been moving out of the cube.

Mill will still exist as a way to apply extra pressure in long games, but it won't exist in the same space that it used to. It isn't a strategy on its own anymore; but rather a way to force players to act, a payoff for another gameplan, or as a major bomb to ramp into.