Southern expansionists wanted to add more territory to the US so they could spread the pro-slavery territory over the slave-free territory. Because of that, they hoped to bring Cuba and other territories to the US and enlarge the empire of slavery.
Also, during that time, Haiti had a slave revolt and overthrew their masters and created a black republic. The Slaveowners did not want that since they wanted to expand pro-slave territory.
Also, they believed that Britain was interested in the Island because of sugar and since Britain prohibited slavery in their colonies, Cuba would be free.
Answer: Many expansionists believed that the Haitian Revolution could repeat itself in Cuba, leading to the overthrow of slavery on the island and the creation of an independent black republic. Americans feared that the British would seize Cuba, which would render all slaves on the island free because Britain had already outlawed slavery.
Explanation: Hope this helps :)