Since World War II, the U.S. has mostly been fighting asymmetric warfare against insurgents and guerillas. This has caused a paradigm shift in our armed forces, to be more prepared for these types of conflicts, for street-to-street fighting as in Fallujah, and wary of threats like IEDs and suicide bombers.

Perhaps our pivoting has left them less prepared to fight conventional warfare on open battlefields against symmetric adversaries? I don't know.