You're absolutely right that improved leadership and governance has much greater impact than the typical EA / GiveWell causes, e.g. bed nets.
However, the problem is we don't know how to improve leadership and governance, with "we" being all of development academics and practitioners. Indeed, the World Bank and countless NGOs have been working for decades, spending billions of dollars on improving governance with little impact. Because of this poor track record, I'm suspicious that spending yet another 1 million on Haiti's governance wouldn't have any impact.
As someone who studies economic development and works in international development for 2 years, I'm quite jaded. The pie seems large, but it's entirely in the sky. In contrast, the EA / Give Well approach only makes small promises, but it surely delivers. That's the appeal.