There were multiple points of failure.
This NY Times article gives a blow-by-blow of what went wrong in the process and systems that were established to protect the church, Notre-Dame came far closer to collapsing than people knew. This is how it was saved.
You can read the article, but both the smoke detection system complicated design and human error contributed to the failure to detect the fire in its earliest stages.
Recently I shared what happened on a U.S. Navy ship that led to a collision with a freighter. The helmsman of the ship that night was the first time the person had ever held that position — ever! In the above case, the person staffing the alarm center at the church was only in their third day.
This is how things happen. Typically multiple factors come into play when catastrophe strikes. Human error is a frequent contributor.