Blizzard isn't the best example to be emulating. They also upset their playerbase by a series of poorly received expansions, and needed to do something to rekindle players' interest in the game. So they chose to kick them in the nostalgia by releasing ancient WoW.
Furthermore, League relies on it's players for more quality games than WoW does. You can have the entirety of Azeroth be composed of like only 3 players and it will still operate the exact same way. League cannot operate in this way, so queue times would be through the absolute roof for ancient mode if most players decided to stick with the new and pretty modern mode. I personally would only play ancient mode if it was like a temporary game mode or if I could earn skins like Silver Kayle, Young Ryze, or Black Alistar, but otherwise, I have no reason to degrade to such hideous graphics and mechanics. It's already a lot that I have to put up with Corki, Cho'Gath, and Mundo in a game where Qiyana, Yuumi, and Sylas exist.