The original source didn't give a reason why Breivik was being held in isolation so I went digging. Not a lot of people are addressing it, but I eventually found a quick blurb at the end of the Reuters article:

The head of Ila prison outside Oslo, a facility where Breivik was previously held, said in 2013 that holding him in isolation was necessary because of the risk he could take someone hostage.

I'm not sure I buy that reasoning. Isn't the whole point of keeping someone in prison so you can maintain security over them? Keep them from hurting others? What hostage is he likely to use in prison and has he already attempted it?