Also, both Haiti and the DR are frequently (~every 5 years?) hit by hurricanes, with earthquakes and flooding also mixed in.
Consequently, if there's a difference in the rate at which infrastructure is built, it's going to more heavily affect Haiti.
Or, to put it another way: a hurricane knocks out a power line that took Haiti 5 years to get in place, vs DR 3. Haiti loses 10 years of infrastructure (initial + rebuild) while the DR only loses 6. To say nothing of the lost economic activity while replacing.
Also I find it peculiar how the damage incurred by the Duvalier family was not mentioned directly in the article.