Fatah. Fatah agreed to a two-state solution and peaceful negotiation, and the Israeli prime minister got assassinated by far-right Zionist loons for his trouble. Ever since, every attempt by Fatah at good-faith negotiation has ended up as demands from Israel for a demonstration of fealty, followed by Israel not following through on its obligations anyway. For obvious reasons, this severely damaged Fatah’s credibility.
Also, Fatah is either hilariously or sickeningly corrupt, depending on whether you feel like laughing or crying on the given day. Hamas used that to worm their way in too, though now they’re no less corrupt.