As has been said before - he wasn't ready.
I had the opportunity to follow him extensively at Michigan, and the kid, frankly, looked like a freshman. A freshman with lots of talent, but a freshman nonetheless. Certainly not as mature as Sidney Crosby or other kids his age.
As recently as the last week of the season, Johnson was sometimes liable on the defensive end, looking too much for the bone-crushing hit and not enough for the smart, defensive play. One of his overpersuits of the body resulted in an OT loss for the Wolverines in their last series of the season (I forget if it was the Friday or Saturday night game... I believe it was Saturday.)
Now, obviously, Carolina wanted him there, but I think it would have been more of a depth move and a move to get him experience on a playoff team for next year. He certainly isn't more capable at this point than any of the Carolina defensive regulars. Yeah, he's got all the talent in the world, but he's just not exhibiting the maturity required to keep a level head in a Stanley Cup run.
His NCAA coach, Red Berenson, more than likely sat him down and told him this, and usually when Red talks, players listen. They've got a great shot to make a deep NCAA run this year (last year they had 11 freshman, including Johnson, Edmonton's Andrew Cogliano, and soon-to-be draftees Mark Mitera and Billy Sauer), and with another year, Johnson will be able to learn the discipline he's going to need to not spend the majority of his NHL stint in the penalty box (like he did through much of this season at Michigan.)
So yeah, if Carolina wins, he doesn't have his name on the cup, but I don't believe he would have contributed heavily anyway (and how much pride do you get from being at the end of the bench... and only for the playoffs?)