Grixis Reanimator relies on the graveyard for Unearth and Persist to cheat back Archon of Cruelty and Emperor of Bones, so Rest in Peace exiling their graveyard on entry cuts off both recursion spells entirely. Celestial Purge exiles a resolved Archon of Cruelty or Emperor of Bones outright since both are black, and Spell Snare snipes Unearth or Thought Scour at mana value two before they can set up. Day's Undoing is dangerous since it refuels a deck that wants a fresh hand of discard outlets and reanimation spells, Isochron Scepter is too slow against their fast graveyard plan, and Force of Negation loses value once Thoughtseize has stripped your hand, so those come out.