Why the 19th century Caribbean sugar cane planters feared emancipation?

Answer 1

They'd lose their cheap labour source. Also, they were afraid that free slaves might take a bit of revenge.

Answer 2

The earlier response is right on. Refined sugar was very hard to make; it required a LOT of labor. If the planters had to pay laborers, they would never be able to make the lucrative profits that supported their lifestyle. Thus, maintaining slavery maintained the colonial economy. They also did indeed fear revenge; they knew slavery was not the most moral system ever devised.