The Steeler defense gave up way too many ill timed big plays. Giving up a 70 yd drive and several 50+ yd drives. Yes they stepped up in the end forcing the FG and giving the offense a chance for a late rally, but they gave up a lot in the first 58 minutes.
You normally like our defense when they are behind the 8 ball and have to make a play. They couldn't come up with any turnovers, which is very uncharacteristic. On the one INT and the Mendy fumble, you figure they are going to atleast force a FG or maybe get off the field on at least one of them.
You can't place the blame on them squarely of course, but on a day like today you expect them to be able to make a play at some point.