Tradition.

Luck and money, black-eyed peas and collars greens.  They're the same thing.  Since my memories began, my father has always made these from the Joy of Cooking (with his own modifications...most notably, double the bacon).  The house normally is open to everyone, where they come and get their luck and money for the rest of the year.  The peas, since they swell when cooked, symbolize prosperity; the greens symbolize money; the pork represents positive motion, because pigs root forward when foraging.  Have to love tradition!