The differences in difficulty between MMA and boxing come down to this: boxing is a more closed system than MMA. If you're in boxing and just have trouble picking up the actual boxing (punching, punch defense, movement, etc) then you're constantly playing catch-up. However, if you take up MMA and have trouble with the boxing aspect, you might excel at another aspect (kicking, clinch fighting, takedowns, ground grappling, submissions, ground-and-pound, etc). There are plenty of MMA fighters who have poor boxing, but because MMA isn't a punching contest, they can be competitive by being strong in other areas. So boxing is more difficult because it's so specialized that if you're simply not good at punching, etc then you'll have more trouble picking it up; conversely, MMA is more difficult in some respects because you need a BROADER range of skills to be at least halfway good in. If you only focus on boxing, or kicking, or the ground game, then you'll eventually find other MMA guys that can beat you because you're not well-rounded.
By way of analogy, the best sprinter, best high-jumper, best long-jumper, etc are participating in individual events, not the decathlon. They could not participate competitively in the decathlon because they've become so specialized; likewise, a decathlete wouldn't be competitive at the individial aspects with the best in the world.
http://en.wikipedia.org/wiki/Decathlon#Decathlon_bests