Grixis Reanimator's maindeck runs Spell Pierce, which counters any of your noncreature spells unless you pay {2}, on top of Thoughtseize stripping a key piece from your hand before you can combo. Silence removes their ability to interact with Spell Pierce (or sideboarded Mystical Dispute) during your combo turn, letting your ritual chain resolve freely. Their sideboard also adds Damping Sphere, which surcharges each spell you cast after your first that turn — a serious problem for a storm plan — and Brotherhood's End destroys it outright since it's mana value 2. Their clock is fast (Unearth or Persist into Archon of Cruelty, or Abhorrent Oculus off Psychic Frog card selection), so treat this as a race and prioritize resolving your combo over grinding out card advantage.