Izzet Spellementals wins with big blue creatures like Eddymurk Crab and Sunderflock, which get cheaper and more threatening as their graveyard fills with instants and sorceries. Ashiok, Dream Render disrupts that plan: its -1 exiles their graveyard, which hurts Eddymurk Crab's cost reduction and denies them resources. Boomerang Basics only temporarily answers their large bodies, since they can simply recast them (and Sunderflock's enter trigger would happen again), so one copy is trimmed to make room for Ashiok. The rest of the deck, including This Town Ain't Big Enough, stays as it is.