Why is Haiti vulnerable to more earthquakes?

The tectonic factors that create the Caribbean islands and haiti are also what causes them to be at risk. Haiti is at the intersection of the caribbean plate and the North American plate with a sinistral (left lateral) strike slip fault to the west and a convergent subduction boundary to the East. Stresses from this oblique convergence will cause earthquakes along the near surface due to the strike slip fault and deeper plutonic earthquakes due to the subduction zone.

Why is there a large amount of damage from earthquakes in Haiti?

Their buildings aren't made of strong enough materials to withstand it. They also don't have the funds to properly build them.