If an airline were just beginning international service from the U.S., to be frank BZE is probably not going to be in the top 20 or 30 choices due to:
lack of local people traveling (the Belize population is so small relative to many other markets),
relatively low levels of international tourism,
relatively slow growth of tourism,
high costs and high hassles for the airline and its ground staff in Belize City,
a reputation (deserved or not) for government incompetence and corruption, and
the checkered history of other airlines flying to Belize (e.g. seasonal cutbacks or discontinuances of service by Delta, US Air and others).