For reasons beyond blaming a lone item or culprit, the Penguins have not been a very strong defensive team this season. A stretch of injuries damaging the blue line shouldn't be ignored in the matter, but the fact remains that they're not doing nearly enough to make life difficult for the opposition's attack on a consistent basis.
The Penguins rank 22nd in the NHL by allowing 2.73 expected goals against per hour at 5-on-5 and 27th by surrendering 32.2 shots against per hour. None of the teams with comparable figures in either metric are threats to go on a run in the postseason.