Totally speculating, but from the outside it seems like stagnation & resistance to change. The Corvette still uses leaf springs. All three of the Big Three hung on to SOHC & other relics for far longer than Japan. I also remember as a shade-tree mechanic, Japanese engines were always precise, intricate, and sharp (suggesting computer design & milling) while the Fords of the same decade were not, and frequently the arrangements were irrational. Rangers needed the intake manifold removed to access the spark plugs. This always suggested to me more casting & hand drawing.

American automakers seem to make some great stuff when they are actually pushing the envelope- the EcoBoost line I hear is very solid, and the aluminum frame in the F150 seems to be doing great (notice: lots of FUD when it launched, silence ever since). The Volt & Bolt have had many great things said about them.

Whether it's conservative engineers, or tight purses, or unrealistic schedules, or even just myopic American Exceptionalism, I do not know.