On the eve of yet another foreign occupation of Haiti,I thought that it was a good time to answer this question.
There is no one reason, and I don't want to downplay the responsibility for the corrupt Haitian elite.
However, we should not ignore the responsibility of the international community either.

First of all, the Haitian revolution was horrific. Around a third of everyone in Haiti died. The country was absolutely devastated. Afterwards, no major nation wanted to recognize Haiti because it might inspire their owns slaves to revolt. That includes the United States.
Haiti was unable to trade for things they needed to rebuild. It was in this environment that a huge French Navy showed up and demanded that the former slaves compensate their former owners, or they would destroy the tiny nation. In 2022 dollars it totaled about $32.5 Billion and took 122 years between 1825 and 1947 to pay it off.

This much you probably knew.
Here's what you haven't heard.
Stealing the poop:

For more than 160 years, the United States and Haiti have disputed the ownership of tiny Navassa Island at the southwest entrance of the Windward Passage covered with what was once worth a king’s ransom. More than a century later, the question remains: Who owns the poop?

Known as La Navase in French, the pear-shaped island is located about 35 miles west of Haiti’s southern peninsula, 85 miles northeast of Jamaica and 95 miles south of Guantanamo Bay, Cuba. Covered in bird poop and managed as a national wildlife refuge by the U.S. Fish and Wildlife Service, it is claimed by Haiti and included in the very constitution that President Jovenel Moise is currently trying to rewrite.

The Guano Islands Act of 1856 allowed adventure-seeking Americans to claim any abandoned or unclaimed islands with guano — the highly valuable 19th-century compost that comes from the excrement of seabirds and bats — on behalf of themselves and the United States.

So this "unclaimed" island just happened to be off the coast of Haiti, but that's just the start.

Safe Keeping Haiti's gold:

In 1914, the Wilson administration sent U.S. Marines into Haiti. They removed $500,000 from the Haitian National Bank in December of 1914 for safe-keeping in New York, thus giving the United States control of the bank.

If that sounds like a bankrobbery, it's because it was.
How did they justify this? They didn't bother. It was all a matter of Wall Street greed.

In the drowsy hours of a December afternoon, eight American Marines strolled into the headquarters of Haiti’s national bank and walked out with $500,000 in gold, packed in wooden boxes.

They drove the loot by wagon to the shore, past American soldiers in civilian clothes who kept watch along the route. Once at the water, they loaded the boxes and sped to an awaiting gunboat.

The gold was in the vault of a Wall Street bank within days.

The operation took place in 1914 — a precursor to the full-scale invasion of Haiti. American forces took over the country the following summer and ruled it with brute force for 19 years, one of the longest military occupations in American history. Even after the soldiers left in 1934, Haiti remained under the control of American financial officers who pulled the country’s purse strings for another 13 years.

Invading Haiti was necessary, the United States said. The country was so poor and unstable, the explanation went, that if the United States didn’t take over, some other power would — in America’s backyard, no less. Secretary of State Robert Lansing also portrayed the occupation as a civilizing mission to end the “anarchy, savagery and oppression” in Haiti, convinced that, as he once wrote, “the African race are devoid of any capacity for political organization.”

But decades of diplomatic correspondence, financial reports and archival records reviewed by The New York Times show that, behind the public explanations, another hand was hard at work as well, pushing the United States to step in and seize control of Haiti for the wealth it promised: Wall Street, and especially the bank that later became Citigroup.

Under heavy pressure from National City Bank, Citigroup’s predecessor, the Americans elbowed the French aside and became the dominant power in Haiti for decades to come. The United States dissolved Haiti’s parliament at gunpoint, killed thousands of people, controlled its finances for more than 30 years, shipped a big portion of its earnings to bankers in New York and left behind a country so poor that the farmers who helped generate the profits often lived on a diet “close to starvation level,” United Nations officials determined in 1949, soon after the Americans let go of the reins.

So by the late 1940's Haiti hadn't made any progress at all, but it was because France and the United States wanted it that way. Specifically, the bankers.

After that came French bankers, dangling loans before a country that had been depleted by decades of paying France. They took so much in commissions, interest and fees that, in some years, their French shareholders’ profits were bigger than the Haitian government’s public works budget for the entire country.

Next were the Americans, at times portraying their intervention as a way of defending Haitian “sovereignty.” And just as it had for generations of Parisian bankers, Haiti proved profitable for Wall Street. In its filing to the Senate Finance Committee in 1932, National City Bank said it secured one of its largest margins during the 1920s from a debt it controlled in Haiti.
...
But according to nearly two dozen annual reports published by American officials and reviewed by The Times, a quarter of Haiti’s total revenue went to paying debts controlled by National City Bank and its affiliate over the course of a decade — nearly five times the amount spent on government-run schools in Haiti during that time.

During the U.S. occupation they created the Tonton Macoute, just to make sure that Haiti didn't recover on its own.