If memory serves, I think the Haitian ex slaves ruled themselves after massacring the French colonials while the Dominicans developed under Spanish rule for much longer and were better prepared to self govern. I think.
That’s right based on my memory and a quick scan of the wiki page. Some other factoids — DR has had the fastest growing economy in the western hemisphere for the last several years with annual growth of around 5%, their electrical power system is 110V/60Hz which means US electronic items can plug in and work with no modification, DR has a small community of English speakers that was founded by ex-American slaves long ago.