I think it has to do with human nature. We all want to be able to have our own identity and culture, so by saying you are a "hardcore" bodybuilder you can identitfy your self with your fellow meatheads. It is just like emo kids, goths, frat boys, etc.
By using the tag line hardcore it makes the insecure bodybuilder fee like he "belongs" to a group. Lets face it, in all honesty, most people start lifting weights because of feeling of inadaquicy. They want to get big to get the girl, or beat up the bully, or make the team, etc.
Fortunately bodybuilding can boost your confidence and make you a better person, unlike, say being a gay crying emo kid.