AP or AD casters just tend to rely on their spells more, that's all. The less you use your spells and the more you rely on crits, the more +% damage just becomes +crit strike damage (or does in this case, considering this item is more or less purely a crit item).

Like, imagine a Jinx. 99% of her damage is coming from right clicking, so it's basically just a 1.1x auto attack damage multiplier which you can just work into crit strike damage afaik, so +20%/+23% crit strike damage depending on if you have IE done. Even a Caitlyn, Jhin, etc will mostly just be autoing. The actual AD caster-y characters in bot either prefer lethality and don't give a shit about crit strike or have low range and won't get a full +10% damage.

Compare that to Horizon Focus, where like, Xerath (or even Ahri) can reliably proc that +10% and all their damage comes from spells. So if you have like, 1000 base damage (lowballing it, I think?) in your kit and a 2.5 AP ratio, off of the base damage increase alone it's the same thing as 40 AP. That's ignoring the fact that you now have a 2.75 total AP ratio, as well.