The linked article says why, but to paraphrase, the training has always said to flip both switches. Previously the only time you’d need to flip the switches was due to an electric motor being stuck-on. It’s only with the MAX that there was an actual useful reason to flip one vs the other, but the training was always to flip both and Boeing decided to simplify it.

My personal guess is that it was internally totally disparate teams inside Boeing and neither communicated about the change. Is that bad? Certainly. But how often does that happen in big companies? All the time. Sadly it might have had a part in causing fatalities in this case, but doesn’t make it any less common.

Either way, it’s not like the change definitively had any bearing. Turning off all electronic control was still the training given to pilots and then reverting to manually turning a wheel that controlled the horizontal stabilizer via cable. It’s not an easy thing to do (turn the wheel manually), especially during a nose dive, but it’s been done before in an emergency and pilots are trained on it.

Ultimately, the FAA had a prestigious record due in part to having excellent checks and balances. Whoever decided to move away from DERs to ARs ultimately should get the brunt of this blame, but sadly, that’ll never happen.