1. External debt
Sure Haiti finished paying France a decade before its economy began to diverge with the Dominican Republic. But what could it have been spending this on for the over a hundred years it was repaying this sum? If the Dominican Republic didn't have such a (frankly ridiculously unfair) debt, is it possible it was able to make some investments in infrastructure, for example, that Haiti wouldn't have been?
2. Land mismanagement
The idea that Haitians deforested their part of the island, causing themselves environmental (and therefore economic) problems is is a fairly common myth. It seems that overall the forests are relatively sustainably managed and Haiti has a level of forestation you'd expect: https://www.vice.com/en/article/43qy9n/one-of-the-most-repea...