I think pilot training is almost certainly a factor. Technically, Boeing are correct that a fully trained and competent 737 pilot should have been able to execute the runaway trim procedure (which is a memory item) and recover the aircraft without incident. However, the fact that two sets of pilots failed to do this in quick succession showed that pilot training could not be relied on to this extent in practice. On average, pilot training ought to be better in the US than in many (not all) other countries.