There is no one reason. There were specific examples of awful line play. I detailed some of those in threads over the course of the season. There were lots of times where DR got sacked because of missed assignments or the line's inability to establish a pocket. Why those happened is also complex and includes coaching, injuries, experience, scheming, etc. Play calling and receiver separation is also a factor.
DR similarly doesn't get a pass. While there are plenty of examples of the things mentioned above, there were also plays where pass blocking was adequate and he simply held onto the ball for too long or didn't get out of the pocket. There were times where it seemed like he wanted to wait for a wide open receiver, rather than ask his receivers to make a play.
Line play has been a problem for a long time now. Adrian Martinez ran for his life for four years. Casey Thompson got teed off on until he broke down. It's not like things were amazing in pass protection prior to DR and he made it stink.