https://www.wsj.com/articles/regulators-found-high-risk-of-emergency-after-first-boeing-max-crash-11564565521

«When the FAA determines an aircraft poses an unacceptably high safety risk, it typically mandates targeted equipment changes, inspections or training to alleviate the hazard. It is unusual for the agency to conclude that reiterating cockpit emergency procedures or tweaking manuals will suffice.

….

It isn’t clear why Boeing took longer to finish MCAS changes than some industry and FAA officials had expected.»

It’s clear to me, a software developer.