Why did devotion decks dominate PT Theros?
Conventional wisdom is that multicolor decks are optimal in competitive Magic due to their broader access to powerful cards


Part of it is that devotion breaks that conventional wisdom, because it actively rewards you for playing lots of cards with identically colored mana symbols. The power level of the main devotion engines &mdash; Master of Waves, Gray Merchant of Asphodel, and Fanatic of Mogis &mdash; is high enough (when you have significant devotion) that it overwhelms the benefit of running multiple colors. For example, if you already have 5 devotion to blue, then Master of Waves gives you 6 2/1 creatures for four mana. Nothing in RTR block is nearly that good, even when you can put multiple colors together.

Of course, the high potential power level of e.g. Master of Waves wouldn't matter if you had to run otherwise terrible cards to enable it. But the cards that have lots of colored mana in their mana costs, like Nightveil Specter and Boros Reckoner, tend to be reasonably powerful anyway, which is related to the fact that they're normally so hard to cast. Devotion was just the extra benefit these cards needed to elevate them from fringe playables to core cards of the format.