Izzet Prowess is a fast, low-curve burn/tempo deck (Monastery Swiftspear, Dragon's Rage Channeler, Lightning Bolt) that can kill quickly, so Six provides a durable reach blocker that survives their small burn spells and generates card advantage via its attack trigger. Aftermath Analyst is too clunky and slow against this aggressive clock, so it comes out for the more resilient Six. We keep Summoner's Pact and the rest of our lands intact since this is a small, low-risk swap and our mana engine remains the priority to race or stabilize against their aggression with Primeval Titan.