The reason is expediency. Getting the ban in now, maybe bringing it back with a nerf later, is good since it fixes games in the short term.
I'd be great to see Stealer of Souls work like the Battlegrounds minion Prophet of the Boar, and that sort of once-per-turn triggered effect would be really cool to see in the game. But I'm not coding these games. The delay to decide on exactly which nerf, and implement it without bugs, might be longer than we'd prefer to wait for a broken (not necessarily OP, but certainly game-breaking) card.