Pyroclasm's 2 damage wipes out Jackal Pup, Grim Lavamancer, and Mogg Fanatic, all 1-toughness or low-toughness creatures that make up a large part of Sligh's early pressure. Swords to Plowshares is premium removal against Ball Lightning, preventing its 6 trample damage entirely since it exiles before combat damage. Wasteland and Armageddon punish their land-light, burn-heavy manabase, especially since many of their burn spells (Fireblast) rely on having Mountains to sacrifice. Call of the Herd is too slow to race a deck this fast, so it comes out for the extra sweeper.