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.
When one of these young men starts reaching into a bag, the other does not recognise the danger he is in, and does not react accordingly. Police pay special attention to this kind of behaviour because they may be reaching for a weapon.
Police in this video clear a house room by room with a mix of best practice and sloppier methods (in particular brazenly opening doors and sticking their head or guns in) before they are confronted with a man who attacks with a knife. Luckily, in the last room they did not approach the closet door immediately and open it, allowing enough of a reactionary gap to avoid being stabbed.
A fairly inept man drops his knife and is controlled effectively when this woman pulls his hoodie over his head. This gives her handles to control his posture, obstructs his vision and limits the mobility of his arms at the same time.
Suspect attempts to flee when a police officer searches him. As the struggle begins, a large knife can be seen tucked in to the rear of his belt. The knife falls to the ground as he escapes the grip of the cop. As the cop again grabs the suspect and pulls him to the ground, a bystander who apparently has some training in grappling assists by controlling the legs of the suspect. The bystander then transitions to a rear naked choke which effectively restrains the suspect who stops resisting.