There are at least two answers to this question. One is informative and enlightening; the other is unbelievably stupid.
According to Seth Borenstein of the Associated Press, Haiti's bad luck is attributable to "a killer combination of geography, poverty, social problems, slipshod building standards and bad luck, experts say."
With regard to geography, Haiti is prone not only to hurricanes and other tropical storms, but also to earthquakes, according to Borenstein.
Haiti's social problems make natural disasters even more difficult to deal with. "It starts with poverty, includes deforestation, unstable governments, poor building standards, low literacy rates and then comes back to poverty," writes Borenstein.
Some, however, disagree with the experts.
According to Pat Robertson, Haiti's suffering is attributable to its being cursed:
This is one reason why I am not a Christian. When some Christians attempt to make sense of reality, they sound like time travellers from the 16th Century. Not only is Robertson's apparent ignorance of 500 or so years of scientific progress breathtaking, but the possible moral implications of his view are repulsive and sickening. Is it wrong of us to send aid to Haiti? Should we all just turn to God and pray and leave it at that?
ROBERTSON: [S]omething happened a long time ago in Haiti, and people might not want to talk about it. They were under the heel of the French. You know, Napoleon III and whatever. And they got together and swore a pact to the devil. They said, "We will serve you if you will get us free from the French." True story. And so, the devil said, "OK, it's a deal."
And they kicked the French out. You know, the Haitians revolted and got themselves free. But ever since, they have been cursed by one thing after the other. ... They need to have and we need to pray for them a great turning to God.
All of you morons who send money to CBN and their propaganda show The 700 Club need to stop. Pat Robertson is evil. By supporting him, you participate in the evil and are thereby complicit in it.
If there is a hell, it is intended for people like Pat Robertson, not me.