Yeah, I think IEDs were the big issue that was scaring the American soldiers. But, that doesn't necessarily mean that the US planners were caught off guard. It might just mean that they thought that an occasional light vehicle taken out by an IED was an acceptable loss.

The number of acceptable losses according to military planners is always going to be much higher than the number of acceptable losses according to the people getting killed. Often the power rests with the planners and the soldiers just have to grumble and accept it. It's interesting that in this case the soldiers were able to make their case so effectively that the military started putting armour on light vehicles, something that's very rarely done.