'Dallas' hit a chord back in the late Seventies and Eighties because it was the age of greed: here you have this unapologetic character who is mean and nasty and ruthless and does it all with an evil grin. I think people related to JR back then because we all have someone we know exactly like him. Everyone in the world knows a JR.
When we read stories of heroes we identify with them. We take the journey with them. We see how the obstacles almost overcome them. We see how they grow as human beings or gain qualities or show great qualities of strength and courage and with them we grow in some small way.