As a general term for penis, I feel that "cock" is more common in the middle of the US, whereas "dick" is the go-to term on the coasts, particularly the west coast. Both terms are known everywhere (and "dick" has its secondary meaning of an unpleasant person everywhere), but I think it's relatively rare in, say, California to hear, "My cock is really sore today," whereas that would be the default word in, say, Minnesota.
This theory probably has a million holes, particularly in the modern mixed-up era, but that's been my lazy linguistic theory for decades.