The Haitian Constitution includes language about "slaves" and "slavery." Why would the writers of the Haitian Constitution include this language but

not the French Declaration of the Rights of Man and of the Citizen?

Because the French were never enslaved, unlike the Haitians.

Explanation:

Haiti was a territory enslaved by the French and the Spaniards and for that reason, its constitution is punctual around slavery, since this was a situation that Haitians had to go through. However, France was never enslaved and they do not know the problems that this type of system causes, as the French never suffered enslavement, but on the contrary they caused slavery in other territories, the French did not include an agenda on slaves and slavery in their declaration of human and citizen rights.

either b) they were often feared and despised by other native cultures. or

c) their obsession allowed them to coexist with many other native cultures because of their peaceful nature.