Selesnya Landfall floods the board with cheap, low-toughness creatures (Llanowar Elves, Sazh's Chocobo, Poison Dart Frog) that scale off triggers like Mightform Harmonizer, so a mana-sink sweeper is premium. Day of Black Sun for X=2 or 3 strips abilities and destroys their entire early board, denying counters and landfall payoffs in one shot. The extra Maelstrom Pulse gives you additional unconditional removal for nonland engine pieces like Earthbender Ascension, since this matchup is decided by whether their engine pieces stick; Flashfreeze and reactive card selection matter less against a low-interaction creature deck.