Cards like Kenrith, the Returned King or The Ur-Dragon demand multiple colors of mana
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
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
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
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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