Just got my first Hellfire ring and switched it out for my Wailing Host. Did a key run and only got like 4-5 rares from the whole run. I'm assuming it's from the lack of set-bonus from the Littany/Wailing combo.
Luck/chance/randomness. 15% is not that different. Sometimes I play for an hour with my low DPS, low paragon Witch Doc and find a few legendaries. Then I play for an hour with my high efficiency, high paragon barb and find nothing but crap.