The flavor of this color-pairing is all about Spider-Man's most iconic villain: the Green Goblin
With cards like Dromokas Command and Silumgars Command on deck to make sure you never actually lose the game, both Abzan mid-range and U/B control might be able to find reasons to run this
Limited availability, stunning artwork, and competitive playability contribute to the increasing demand
It's not just creatures, either - Nahiri the Harbinger is a good example of a card that rummages every turn and has a big payoff, also
The [c]Circle of Protection[/c] cards are a mega-cycle of white enchantments that each cost two mana, 1 white and 1 generic
As a cautionary note, unless youre in dire need of cards, only play Edgewall Innkeeper out when you know it will replace itself