I think the scrap armor they're talking about adding was to deal with the huge amount of mines they were dealing with in Iraq.
Speed doesn't save you from an explosion.
The US went in with a mentality and approach, and the resistance forces found a weak spot and leaned on it.
Honest appraisal and adaptation is necessary in any competitive activity, not just war.
I think it's the "honest appraisal" part that the US can't do.