To pirates, merfolk are a threat
Baleful Strix Grima, Saruman's Footman Dimir Infiltrator Master of Death Lazav, Dimir Mastermind Halo Forager Obsessive Stitcher Araumi of the Dead Tide Etrata, the Silencer Tomebound Lich Cavern Harpy Dimir Guildmange Tidehollow Strix Oonas Gatewarden Aven Hearstabber Big Monsters Dimir doesn't have as many large creatures as other colors, but it does have a few
While there are fewer colorless commander options available than there are commanders of other color identities, there are still several powerful legendary creatures available for those looking to construct a colorless commander deck
Because Set Boosters sold better, they chose them
Potentially getting Treasures off this card is what makes it so strong
The TLDR version is this: build a near-infinite engine to gain excellent card draw and play increasingly useful cards on early turns