As you indicated in passing in the question itself, the key to the matter lies in the phrase 'access to powerful cards'. Often (though keep in mind that standard environments with very few multicolor cards aren't that uncommon; think of the pre-RTR environment!), the powerful cards are multicolor cards like Sphinx's Revelation; but their being multicolor isn't specifically correlated with their being powerful. There are other 'paths to power' for a card, and one that Wizards dips to occasionally is the extra colored mana cost; consider a card like Genesis Wave (or even more prosaically, Leatherback Baloth where the card is 'allowed' to be more powerful in exchange for stronger restrictions on the decks that can cast them. This is essentially the same tradeoff that's made for multicolor cards, an increase in power in exchange for a decrease in availability; the difference is just in how the decrease in availability is handled. Seen from this axis, devotion just serves as another means of enforcing the same restriction, only requiring the strong committment in terms of permanents/mana symbols in play rather than in the individual spells' casting costs.
There are also some localized metagame factors that particularly contributed. First, mono-red aggro decks were a major part of the pre-Theros metagame, and so cards like Frostburn Weird (as an early blocker that can eat many of the mono-R creatures) and Master of Waves (as a protection card that's almost impossible to get through even without the additional bodies that he brings) are solid metagame choices against that deck even without the devotion aspects.
Secondly, and more broadly: in an 'unknown' environment, particularly a post-large-set new environment, new decks are generally likely to do better than established decks, because it's much easier for the new decks to prepare for an established metagame than for the established decks to figure out which of the new decks are most critical to adapt to. This state of affairs often doesn't last long, though, and once the new decks are better-understood then the existing decks quickly learn which tools are most important for combatting them. This can be seen in the post-PT-Theros metagame, where Esper has taken over as the control deck of choice and WUR has completely evaporated; black removal like Hero's Downfall has proven to be a better option than red burn since it can successfully hit cards like Master of Waves and also conveniently hit a Domri Rade that's already added loyalty. Even the new wave of Mono-Black decks are essentially a reaction to the 'first wave' of devotion decks.