Izzet Delver is racing us with Dragon's Rage Channeler and Murktide Regent while backing them up with Daze, Force of Will, and Wasteland, so we want efficient interaction that dodges counterspells and attacks their mana. Abrupt Decay cannot be countered and cleanly kills Dragon's Rage Channeler or a Cori-Steel Cutter token at instant speed. Force of Vigor blows up Cori-Steel Cutter for free by exiling a green card when it isn't our turn, which is a huge tempo swing against their equipment plan. The extra Barrowgoyf gives us another deathtouch, lifelink blocker that grows quickly once cards start hitting graveyards, helping stabilize against their aggressive clock. Once Upon a Time and Sylvan Safekeeper come out since they are too slow against a deck this proactive and do nothing to answer their threats.