The trend is that airlines start service to Belize with a big splash, but then when the reality of the small and highly seasonal Belize tourist and business market sets in they pull back. For example, USAir started a few years ago with daily two-class (First and Coach) service from Charlotte to BZE. Now they're down to weekly one-class service much of the year. Likewise, Delta started with daily two-class service from Atlanta. Then they went to a small regional one-class jet on weekdays, with two-class 737 service on Saturdays only, and now I'm not sure what they're flying. Delta was going to do LAX-BZE service, but that fizzled. Despite its limitations, CO's service seems the most consistent on a year-round basis.
I dislike MIA and AA there so much I've given up flying through Miami to anywhere in Central America or South America unless I absolutely have to (which unfortunately is often a necessity).