Why was Sona disabled in ARURF

You forgot that Garen stacks his E on top of eachother and that his W gives him 80% damage reduction, which the cooldown is over by the time he casts it again.

I'm guessing because her ultimate gives her basic abilities up to 40% additional CDR. I'd be a little hypocritical to not let you take Ultimate Hunter or a similar rune, while Sona runs around with one built into her kit.

For the same reason League is in the current state overall.

They just... can't.