After the 2017 USS John McCain and USS Fitzgerald incidents, which were blamed in part on irregular watch systems leading to extremely fatigued crew, the USN is working to adopt watch rotations based on natural circadian rhythms, to promote higher quality sleep for sailors and reduce chronic fatigue (entirely anecdotal, but on a recent Fleet Week tour of a ship in my hometown a few sailors I talked to considered it to be a significant improvement)

My question is, why on earth would it take so long to adopt that sort of system? The problems posed by the previous watch system were notorious and have led to accidents in the past, and the fix seems pretty simple - why did the old system persist for so long? Or was there something in recent years that made the problem worse that wasn't present, or at least present as much, before?