Cost of living and the weather, to some extent. Dominican born Mayor Willie Lantigua of Lawrence, MA, stole a brand new garbage truck and sold it in the Dominican Republic. Lots of places in New England are more Hispanic than white.
I'll bet that many Domincans come into the US under pretense of being Puerto Ricans. Have you heard of this? It wouldn't be too hard to travel from Dominica to Puerto Rico since they are close, and then hopping on a plane from Puerto Rico to the US.