I've been told its a combination of a better dispatch rate compared to the 320 series, combined with the fact that most of those locations used 3rd party MX. So when the Dominican mechanic does need to fix something, avionics bay is ground level and lots of ability to pull CB's to reset a system, where as the 320 sits higher up and resetting things can be more involved than a simple turn in off and then back on kind of thing.