User blog comment:Zilla4444/AP Version of Black Cleaver/@comment-76.27.106.155-20120619221912

ap inst ad

auto attacks of every champ fall within a small base attack speed range and a small base ad range, while ad spells that apply on-hit effects are very very few

while "on-spell" effects are game breaking as every champion has 4 different spells and each one has vastly different base stats, base cool down and most of all differnt ways it applys damage, riot has many reasons they tread very very lightly in this area,

they insted give ap items active/arua effects, if u really want more mr reduction choices, ask for a item w/ a skillshot that would be far more interesting, more balencable ,and viable on more champs