when did yuri become so insanely popular on /a/

Yuri = Purity
When a girl yuris it immediately tells the audience this girl will not be taken nor stained. She will maintain her idol-like anime girl image and never fall for a bland male MC (not you) or a random NPC. You can love her all you want without fear of being a cuck.

Lesbian is literally the perfect form of an anime girl. Imagine having a girl who shares the same interests as you. Secretly drooling over another girl... it's so cute. Conversely, imagine liking a girl who's fond of men. You can't relate, you can't understand. It's bad. It's gay. Cuck-coded by design.