probably because gunslingers statistically don't perform well, and since riot is riot, they throw random buffs at them instead of figureing out why gunslingers don't do well.
The reason why gunslingers aren't great overall is that they are heavily item reliant. If you get the needed items for them, especially guinsoos+either crit or on-hit, they can totally destroy. But if you just don't get those items, they fall of hard mid-late.
And with the addition of sparring gloves, getting the right items became less reliable, so gunslingers do well less often.
Thing is, with that PBE buff, nothing will changes about that. They will hardly perform better without the right items, they just will be more dominant if they get the right items. It's a pure win harder buff that won't really help them do well more reliably.
Also, gunslingers are to RNG reliant. Just had a game with a double-RFC+IE 3star graves, and dropped out first casue two fights in a row he would neither crit nore get a multi hit till he dies...