While I doubt this gorgeous Xerneas card will get that kind of hype, its one of the top IR chase cards revealed from the set, featuring a memorable Kalos Pokemon that fits the theme, painted in a decidedly mid-1900s animation kind-of-way
While it can help fuel colorless mana and provide some flexibility, its entry tapped and limited competitive applications make it less appealing in most formats
Cards like Kenrith, the Returned King or The Ur-Dragon demand multiple colors of mana
The second reason is that the player base of Magic as a whole, and the players getting into Modern and Legacy specifically, has only increased since Modern Masters was released
Moonshadow Remarkable value for just a single black mana, Moonshadow is a 7/7 creature that comes into play with six -1/-1 counters on it, Alternatively, Moonshadow is a great target for Blight counters , as it can simply remove them when you sacrifice other creatures, or lose creatures
It plays best in landfall decks because it puts so many lands into play all at onceperfect for overwhelming your opponents in a single turn