The main culprit is the huge power creep that MTGs had across the last few years, printing cards especially creatures that are more powerful than old Snappy in a vacuum
Getting to put a Wondrous Crucible or Bolas's Citadel into play well ahead of schedule while still having all your mana is incredibly powerful
Aggressive Mammoth If you're running a big creature strategy, Aggressive Mammoth is your dream come true
Whats in the Box There are various ways you can approach Wilds of Eldraine
On top of a critical mass of Myr, like Alloy Myr, Myr Retriever, Hovermyr, and Myr Turbine, green does add a few tools to the deck
Wretched Bonemass Color Indicator: Black Creature Skeleton Horror Wretched Bonemasss power and toughness are each equal to the total power of the exiled cards used to craft it