The Taíno, the original inhabitants of the island, were wiped out / assimilated by the Europeans who settled the island. Years later the Spanish and criollo settlers relocated to the east of the island leaving the west free for the French to develop.
The French and the Spanish had different cultures and radically different strategies on how to develop their colonies and thus the island ended up with two different cultures and governments and languages and political systems and with people of different (but overlapping) racial composition.
A lot of people wrongly assume that Caribbean is an ethnic group but that couldn't be farther from the truth. I'm a Puerto Rican and I have a lot in common with Dominicans and Cubans because the three territories are former Spanish colonies. But I have little in common with Haitians, Jamaicans or the inhabitants of the smaller islands of the archipelago. To illustrate my point, I have an archipelago 40 miles east of where I live and we have no ferry between the islands and no connection of any sort beyond the fact that we're both US territories. I've been there as a tourist but nothing more.
Plenty of countries do very well without shared ethnic seeds, whatever that means, including the wealthiest country in the world.
Edit: Thanks for explaining what you meant, below. That makes sense.
I explained that they're different because they're comprised of different people with different histories.
This might be obvious to you but in my experience a lot of people assume that the inhabitants of the Hispaniola came from the same seed, as in they were the same group of people who later split when that was not the case. One country was seeded by the Spanish and the other one by the French.
My comment was not meant to be a thorough analysis on their respective economies.
Simply put, Haiti was highly dependent on international trade for the resources it needed, which meant that the embargoes from major powers crippled the nation.
Mike Duncan has an excellent series on the Haitian revolution in his Revolutions podcast series, which includes a final episode that summarizes the history of the nation up to the mid-20th century.
For fun I ran the numbers, at 4% real interest which is realistic for a developing country, that's 2.55 trillion dollars in lost interest.
Haiti was indebted to France for over 100 years after the revolution. They were forced to pay the losses sustained by the French, which included slaves. https://en.wikipedia.org/wiki/Haiti_indemnity_controversy
It doesn't get more unjust and wrong than that. To say they have no one to blame for their country's condition is misguided.
http://latinamericanscience.org/wp-content/uploads/2014/03/H...
"It has been widely reported that in 1923 over 60% of Haiti's land was forested; the source of this assertion remains unknown but may be linked to the U.S. Marine Occupation in Haiti. In 2006, the country was claimed to have less than 2% forest cover."
https://en.wikipedia.org/wiki/Deforestation_in_Haiti#Estimat...
Spain colonized the East of the island and government and power remained in the hands of European settlers.
Haiti was also occupied by the US in the early 20th century, which was not exactly a glorious episode, either, and certainly did not help.
(You might hope that that quote exaggerates what was done to Haiti, but if anything it underplays it.)