First, PD acquired those players & as a young head coach it's understandable that he is going to play the players his boss brought in especially given they have NHL experience. And I would argue that the team was bad at the beginning of the season more so because of terrible goaltending although the defence has been bad as well. The defence has been better since the goaltending got better & it's just some people's opinion that Brannstrom has outperformed them, what do you base that on? The experts disagree with you & IMO he's had some good shifts & some bad ones, he hasn't stood out as anything better, not like Zub has or Reilly since he's been put with Zub.
Second, the main criteria is to play the best players you have & at the moment Brannstrom is not one of them. On this team like any other team there are only six defence positions although occasionally some coaches will play with 7 D. Four of those positions are covered by Chabot, Zaitsev, Reilly & Zub who are all better than Brannstrom. The other two positions DJ prefers to have defencemen who can also play on the PK or add some size & toughness to the defence corps & that's not Brannstrom, it's not Wolanin either. They have had a rotating crop of Gudbrnason, J. Brown & Coburn, but it's also because that's all he has that have solid NHL experience. It will be interesting what they do with Brannstrom & I expect he will be given a little more rope next season, but at the same time if they sign any new experienced D it could be the same story again.