It not even simple "we needed to blitz more." It was basic assignments. There were numerous times The LB and S covered the same gap on run plays, with the S aggressively coming downhill. So, many times, when the play went A gap, it seemed the only player covering the gap was a DL and when the DL who had that responsibility was beat, double teamed, or trapped, a run up the middle sprang for 15+ yards b/c both the LB and S both aggressively plugged the B and/or C gap. Either that was a gross misunderstanding of assignment/role by the players or the scheme took them out of position. Basically, the way everything was schemed, if we didn't have 100% decision making AND execution, we got gutted for a big gain. There was way too little margin for error from the get go with play design for our defense to be successful, ESPECIALLY with the # of plays they were expected to play. I think it had less to do with tiredness (although that is a factor) as more plays = more times for something to go wrong. And when it went wrong, it went horribly wrong.
This is also why the hallmark of our defense the last few years was to get 2-3 stops, let up a big play, and then make a few more stops/fall apart. There is just too much going on/too many moving parts/too many breakdown points in the scheme.
Cam had a decent post a while back about some of our defensive philosophy and how it compounded our issues - couldn't find it on a search.