Coming from a structural engineering standpoint, the structural integrity of the structures in haiti where not able to withstand the massive seismic loading that was applied to many of the buildings within the country. The columns in many building that where not either properly reinforced or large enough, and simply sheared off, thus the columns would collapse on its self. Many buildings that where constructed with properly reinforcement were able to withstand the seismic loading, and where able to remain intact with minimal damage. The reason for the devastation behind the earthquake in haiti was simply the lack of funds available to construct reliable buildings.
The location of the earthquake to the surface actually makes very little difference to the damage caused by it because practically all the damage from seismic events is from surface waves anyways. (but yes the Haitian earthquake was very shallow) The reason for the damage is as was stated above; it is to due to the extreme poverty and lack of building codes combined with the fact that it hit in the most populated region of the island.
Because (as was said) Haiti is the poorest country in this part of the World, and since the earthquake was so massive, it literally just collapsed the economy and government because all of its industry was destroyed, what little there was, and the government and law enforcement buildings were destroyed. It's especially bad because the economy and government were already very weak, and now they are simply non-existent.
Haiti is the poorest country tin the western hemesphere
Extremely poor infrastructure :\
The country is so poor that they have not a lot of money to build earthquake proof biuldings