Because he was not trusted to handle any of the shutdown situations that the other guys had to. Hainsey and Zaitsev were usually on the ice against top competition in key situations where we needed to prevent a goal, and Rielly was our #1D so even if he was used more offensively, it was against the other team's best guys more often than not.
Defensively, I would pick neither. Both suck defensively, but in their own ways. I think Barrie on any given night is never great, but is still much better than the lows that Gardiner had far too often, while Gardiner is wildly unpredictable with high highs and low lows. I really do not know which I would pick, but if I were his partner, I think I would rather just pick the guy who is simply incapable. At least you know what you are getting on any given night and how to prepare for it, and which parts of the guys' game you know you will have to cover for every night.
When it comes down to it, both need to be heavily sheltered, pure offensive defensemen to be effective, so I am far more concerned with what they can bring offensively and offensively, the choice is easily Barrie. Gardiner's best offensive season barely cracks Barrie's top 5, with this year even being close to Gardiner's best offensive season despite Barrie being practically useless under Babcock for the first like 20 games of this year. Carolina may give Gardiner one more season to get his crap together, or else Gardiner may end up getting Shattenkirk'ed and fighting for his life in the NHL.