Same reasons Americans thank God instead of the people responsible.
No, really. I know a guy who once ran marathons. He got up early, trained, went to work, went training again, came home, ignored his kids, annoyed his wife, went to bed, and then got up to do it all over again. For months. And then he ran his first marathon. To this day, he truly believes that he never ran a marathon, but that Jesus did. None of them. I mean, he only did a few before he decided to leave his wife, shack up with a cougar, get herpes, and move to Indiana, but, yeah. Even after all that, he still believes that he didn't run the marathon.
And if you ever watch American televangelists, you'll find such sentiments are not uncommon. People putting in all sorts of labor and toil, and apparently it's all "God's work", so people send in their money. Because it's God asking, and not other people.
I don't think either of us could, with a straight face, assert that Haitian society is as generally well-educated as, say, American, Dutch, German, or English societies. I find nothing odd that an exceptionally poor and abused people should have a superstitious worldview that resembles the outlook of many of my lesser-educated American neighbors.