UR Aggro's key threats like Nova Hellkite and Colorstorm Stallion cost 3 or more, making Requiting Hex's mana value 2 restriction miss the important targets. Dawn's Truce protects your board and permanents from removal and targeted burn spells on a crucial turn, while Get Lost and Pyrrhic Strike answer their bigger flying or ward-protected threats that Requiting Hex can't reach. Emptiness's -1/-1 counters are too small against 4/5 bodies like Nova Hellkite, so it's traded for more relevant interaction.