Mono Red Rally floods the board with 1/1 tokens and small artifact creatures (Clockwork Percussionist, Rally at the Hornburg tokens) before going for a lethal alpha strike, so a sweeper is one of our best tools. Drown in Sorrow wipes their entire board of 1-toughness threats before they can chain into Rally at the Hornburg or Goblin Bushwhacker for lethal. Tragic Slip is cheap, efficient removal for their small creatures at any point in the game, including their combo pieces. Nested Shambler's tokens don't survive our own sweeper or meaningfully block their go-wide plan, so it comes out.