In part because they have become experts at taking a mess and making it a bigger mess. The Haitian government has over 200 years under their belt doing just that. You can't teach new tricks to an old dog sort of thing.
Man, you couldn't be more wrong. Under Trujillo, the DR trailed Haiti in Per Capita Income by a large margin. I think it was 4-1 ratio. It was under papa doc and baby doc Duvalier that the Haitian government and people faltered which also was during the 50's.
Haiti also lent assistance to other nations like the French Congo during the 60's. (That one didn't work out to well because it most likely resulted in the HIV infection that set off the pandemic in the Americas Europe, and Austria of the dominant subtype B strain.)
Before that time, Haiti routinely invaded and kicked the Dominicans around and looked down on their poor backward neighbors. Times have obviously changed.
And, please, do some research before replying with a snappy reply.