Opponents don't want to "waste" removal on our Sisters, even though we're gaining life for every creature that enters the battlefield (in the case of Essence WardenEssence Warden, Soul WardenSoul Warden, and Soul's AttendantSoul's Attendant)
The goal of this archetype is to flood the board with small, cheap creatures that you can buff for big, early swings
Eldrazi Incursion has 13 cards in the $2-5 range, two in the $5-10 range, and three at or above $10, including Morophon, the Boundless, Eldrazi Monument, and Eldrazi Conscription
For example, if it's discarded, countered, or destroyed, put it into your graveyard
Bruvacs comparative rarity among mill decks is not down to its power, but instead its lack of ability and resulting exorbitant cost