The keystones in the other trees are good, but the individual mysteries in Precision are just so good for offensive champions generally (they are basically the old offensive masteries). However, they are gated for ADCs mostly since Precision gives an attack speed bonus when used as primary.
I mean if you compare the individual mysteries of Domination and Precision it's not hard to see why assassins/fighters are struggling compared to ADCs. It feels MUCH better to use Precision as a primary than Domination - that second tier of masteries in Domination feels practically useless, and the third tier doesn't benefit most champions. Compare Zombie Ward and healing from ability damage to permanent extra attack speed on takedowns and extra damage to low health champion.
There are other reasons why ADCs are so good, such as easy availability of crit, but the main reason they were strong in Season 7 (supportive items being too overbearing) has been removed. So this might explain why they've become even stronger in preseason.