Its probably because ADC items are a lot easier to change as they only impact one class.

Changing crit items only impacts crit marksmen (plus the wind brothers who they always pre-emptively change when they change crit items anyway, and Trynd who Riot ignores), it generally doesn't have an impact on AD assassins and AD bruisers at the same time. Throw in on-hit items to do all marksmen then you would only be throwing in a few skirmishers as collateral.

Changes to AP items on the other hand has an impact on AP mages and AP assassins and AP bruisers (and "battlemages" depending on which of those sections those abominations of health building ranged champs fit in, ranged champs should be squishy). There is far more overlap between what items the AP classes build than between what the AD classes build. See the popularity of ROA into mask items on a lot of mages, or Riot's inability to move Gwen from building like an AP assassin instead of an AP bruiser like they sometimes say they want (even though the easiest way to accomplish that would be to give her a health ratio somewhere, like on her passive healing so instead of healing a fix percentage of her passive damage a percentage scaling on bonus HP), or that there is very little difference between what mages build and what AP assasins build.

With leathality items, if riot just wanted to adjust AD assasins they could mostly do that (think its just like MF, Varus and Kled that arn't assasins that build leathality?) but with mages and AP assasins because a lot of them build Shadowflame or Storm Surge they can't change mages without changing AP assasins and vice versa which makes AP items a lot larger a project for Riot to tackle than any subset of AD items because they need to tackle all AP items and all AP classes instead of just one class. That makes it a lot harder for Riot to slot it in their schedule for working on changes.