Why does Runaan apply the splash damage from Jinx, but not from Kayle?
Balance.
Kayle would be broken with Hurricane spreading her splash.
Jinx is kind of ~eh with Hurricane.
Kayle's AP-scaling magic damage is all an on-hit effect, so it would see 100% strength on Hurricane's side bolts.
Jinx's damage is all AA based, so it will only see 50% strength on side bolts. Unless Jinx builds crit, which Hurricane can't spread, so even less damage.
It's nice to have items like Deathcap, Infinity Edge, or Athene's that have pretty solid synergy with a champion and offer a solid power spike. Hurricane on Kayle though? That's a recipie for game-breaking, or the type of awful balance where Kayle feels really weak without Hurricane, and Hurricane is only cost-effective on Kayle.
Kayle already is one of the few champions that consider Hurricane, spreading her passive was enough to put it in her core build earlier this season.