The explanation is that we are at the very edge of the satellite coverage, and thus very vulnerable to weather, atmospheric disturbances etc.

XM Radio (+ Sirius and other US based Satellite Internet, Radio and TV providers) are not licensed or allowed to transmit outside the US (+ sometimes Puerto Rico), so it can be hard to pick up the signals, which is why i.e. a DirecTV subsciption must follow a US address and requires a 7' dish in most places here, while a 2' dish is sufficient in the States.

It'll get better once Belize gets a legal non-pirate provider of int'l TV and radio signals.