I'm not interested in talking about Greg Clark or making comparisons to the West; if need be compare it to other black Caribbean nations, such as Jamaica or Barbados. It's much worse and in terms of social indicators it is also worse than many places in Africa. Why? Here a few hypotheses (NB: I don't endorse all of them):
1. Haiti cut its colonial ties too early, rebelling against the French in the early 19th century and achieving complete independence. Guadaloupe and Martinique are still riding the gravy train and French aid is a huge chunk of their gdps.
2. Haiti was a French colony in the first place and French colonies do less well.
3. Sugar cane gave Haiti some early characteristics of "the resource curse," dating back to the 18th and 19th centuries.
4. Haiti was doing OK until the Duvaliers destroyed civil society, thus putting the country on a path toward destruction. It is a more or less random one-time event which wrecked the place.
5. Hegel was correct that the "voodoo religion," with its intransitive power relations among the gods, was prone to producing political intransitivity as well. (Isn't that a startling insight for a guy who didn't travel the broader world much?)
6. For reasons peculiar to the history of the slave trade, Haitian slaves came from many different parts of Africa and thus Haitian internal culture has long had lower levels of cohesion and cooperation. (The former point about the mix is true, but the cultural point is speculation.)
7. Haiti has higher than average levels of polygamy (but is this cause or effect?)
8. In the early to mid twentieth century, Haiti was poorly situated to attract Chinese and other immigrants, unlike say Jamaica or Trinidad. It is interesting that many of the wealthiest families in Haiti are Lebanese, such as the Naders.
Overall I don't find this set of possible factors very satisfactory. Is it asking too much to wish for an economics profession that is obsessed with such a question?
If you are looking for some cross-sectional variation to ponder, consider the fate of Haitians in Suriname (they make up a big chunk of the population there), Haiti vs. Santiago, Cuba, pre-Castro of course, or why early Haitian migrants to Montreal have done better than later migrants to Miami and Brooklyn.