Mono-Blue Faeries relies on small evasive flying creatures like Spellstutter Sprite, Faerie Seer, and Faerie Miscreant; Cut Down cheaply answers all of these since each has total power plus toughness under 5. Grim Bauble's -2/-2 outright kills most of their 1-toughness Faeries before they can trigger Ninjutsu or protect with Counterspell. Reckoner's Bargain and Giant's Boulder are too slow against a tempo deck built to out-card and out-tempo us with cheap evasive threats and counterspells.