Sicily was rich in its own right, one of the richer breadbaskets of the Mediterranean, supplying the world with vast amounts of wheat and meat. They also had salt pans, raw hides, cotton and fish for sale, and North Africa had gold to pay for it.

Then there was the location. The island straddles east and west and is a natural trading center between the two. The Christian Normans made a point of being open handed towards Muslims and Jews, and keeping the ports open for transfer of goods - silks and spices, that sort of thing. Merchants were generally expected to pay duties and tolls, another healthy income stream.

Regarding slavery on Sicily at this time, the records are a little sparse, and the references are to domestic slaves. On taking over, the Normans were happy to keep the mosques open and the Muslims at hand. It made for good business. The king could keep Muslim slaves, and there are a number on record. Jews and Muslims were forbidden to hold Christians slaves. The mass of field workers, however, were closer to serfs, which was a bit better than slavery.

The current go-to source for this sort of thing is David Abulafia, who has written a number of books and papers on this time and place. More general reading material can be found by John Julius Norwich. See also the Nroman Kingdom of Sicily by David Matthew.