I wonder if language contributes to it. Here's my logic:
- Haiti is almost the only French-speaking nation in the Western Hemisphere. (Martinique is a part of France, Quebec is a part of Canada.)
- Networks of communication and trade are very important in the development of economies. (Assumption)
- Communication is easier when there's a shared language and harder when there isn't one.
- So over the centuries Haitian people have been at a slight disadvantage in dealing with their potential trading partners in other areas, simply because of language. Over time, that disadvantage could add up.