Kirk was a head case, someone with tons of potential and no desire to train. I remember reading an interview with him once where he said he really didn't like the idea of fighting and that he hated to train. Kind of a strange career move to become a professional boxer if that was true.
You can either show up at 210 and get on your bicycle as David Ibragimov did recently and have the built in excuse "I was too small, WTF I was supposed to do, stand there and fight?" or actually bulk up to compensate the weight difference and get out on your shield with the excuse "at least I tried", the Klitschkos beat you either way. Johnson simply chose the 2nd option, there isn't much to add.