Good question. This is what makes these accidents so bad in my eyes: they weren't caused by a defect or flaw in the traditional sense, but rather by a system that seems to have been conceived solely to allow Boeing to skirt retraining/recertification requirements, to sell more planes.

Such a system should have been subject to extreme scrutiny, and honestly it beggars belief that the potential for these accidents wasn't seen. "This system can trim the nose down beyond the authority of the manual controls, based on readings from a single non-redundant sensor" should have been more than enough to get the gears turning, and it seems that the deeper one looks it only gets worse.