Guyana was originally settled by the Dutch, and then was taken over by the British. Fun fact, just like the Netherlands much of the most heavily cultivated and populated coastal land is actually below sea level. Guyana remained a British colony for 200 years, until 1966. As a result, the primary language is English. No need for interpreters!
The infrastructure of Guyana is also fairly well developed and the social structure is very stable. This means that the planners for the trip have a much easier time organizing. Transportation is not nearly as much of a trial when roads are maintained. Social clubs like Rotary International also have a strong presence and can help in organizing venues to hold the clinics.
So basically, Guyana was the ideal destination because there is a need that we can meet with fewer obstacles.