OC spray and Taser were used to take the man into custody following this. The police officer suffered a 6cm wound and survived.
An ambush gives the attacker initiative. They have had time to mentally and tactically prepare for the situation. The defender may not be psychologically prepared or have any kind of plan, and is at minimum a few seconds behind the attacker to even understand what is happening.
The cop who stood there for a few seconds either did not understand what was happening or was in a kind of denial for some time. This is not a conscious decision made while observing the situation. It is likely a result of inadequate training.