It's for the "greens" part. Some people have collards or other greens; my family always did cabbage. Besides, we just had collards last week. ha
Here's what one site had to say:
"What about Cabbage? Most people will tell you they eat cabbage or collard greens to ensure prosperity. The green leaves represent money. The real truth is closer to this. Cabbage and collard greens are late crops. That means, especially in the south, they would be plentiful and ready for harvest in early January. Fresh veggies meant good nutrition.
The ancient Egyptians, the Greeks, and the Romans ate cabbage. They ate it to ward off disease and aid digestion. If you've ever eaten cabbage you know about that digestion part.
So soak up a mess of peas and cook down some cabbage and steam some rice for New Year's Day and let's celebrate in true southern style. Here's to good luck, good fortune, prosperity and a wonderful new year that we can all share together."