The opponent's reanimator deck relies on resolving huge reanimated threats like Griselbrand, Atraxa, Grand Unifier, or Archon of Cruelty, and the most direct way to beat that plan is to take away its targets. Nihil Spellbomb exiles a graveyard on demand, so it can strip Reanimate, Animate Dead, and Shallow Grave of targets before they resolve. Massacre is left in the sideboard. Their free-cast condition can't be met because it needs an opponent who controls a Plains, and this opponent has no way to produce one. Even hard-cast, its -2/-2 does nothing against their 6/6 and larger creatures, and it would also hurt our own small creatures. Barrowgoyf gives us a lifelink, deathtouch clock that can trade with ground threats like Raph & Mikey, Troublemakers, but it cannot block the flying reanimation targets. Bilbo and Brazen Borrower are cut since their small bodies do little against massive threats.