Even though it gives AD bonus, it also converts 20% of damage to true damage, does not specify only physical. So why not take it on like Fizz, Diana, and Katarina?
Like is it ever a viable choice on them if the enemy is a tanky bruiser ap like Vlad or Swain? Or if the enemy comp has a lot of tanks?
20% true damage alone isn't what makes the keystone good. If your champ doesn't scale with AD it's not worth having.
Becasue summon Summon Aery, electrocute and commit are better options for them than a sustained damage keystone.
The thing is, while maybe it could be decent still, it simply cannot offer more for AP then the alternatives. Conqueror is balanced around the assumption that it increases your damage and your penetration. If you only gives you one of those, then you are getting a far weaker version then was intended. Now if the other keystones were weaker then it, then it could be worth it still. However, many of the other keystones are very powerful, such as electrocute or comet. These keystones are so effective on AP, it would just be supremely inefficient to miss out on them to get half of conqueror.
Those champions are built/player as assassins. While it could work for a bruiser Diana build those champions typically prefer electrocute for the burst damage.
None of those champions even want to be in a fight for longer than 4 seconds. It would never get used.
Those champions want to burst. Akali has always been able to run it due to hybrid ratios as well as innate sustain, but it causes a shift in her playstyle.
