First, we want as many ways to dump cards into our graveyard as possible
It was absurdly good with other fast mana spells, letting you empty your whole hand, and then get a brand-new one
While theyll still be just as rare, with only 500 of a given card to exist, the fact that 100s of these only-500 printings will be created means that finding one for cheap will become relatively easy
On its own, Swarmyard Massacre is strictly worse than Crippling Fear, but as soon as you have a Squirrel or two on the board, Swarmyard Massacre gets pretty scary
But due to the fun-first culture of the format, shutting down someones ability to play their cards even if its partly a consequence of that opponents deck building choices is heavily stigmatized
Some can alter the way combat functions, or how targeting with spells works