While playing around with a Jeeves Warlock zoo deck, I realized that zoo decks have very little reason to run Soulfire instead of Power Overwhelming now that Soulfire costs 1 mana. Zoo usually has an expendable Spectral Spider or a Nerubian Egg as a target for P. Overwhelming. Losing your weakest minion on the board seems like it's often better than discarding a random card from your hand, and zoo usually has stuff on the board in the first place. I guess there are decks that can run both, but having no Soulfires means you never have to worry about tough Doomguard + Soulfire in hand decisions.
Handlock is different because they're okay with having a mostly empty board or one with just Ancient Watcher, so they have reason to prefer Soulfire. Now there's Dark Bomb though.