Factor into this, the fact that it takes a number of people a fair amount of time to get out the exits. While waiting, the natural response based upon exiting a plane dozens of times is to stand up and grab your bag.
To try and get people to leave their bags is counter to the training that they have received and acted upon dozens to hundreds of times on prior flights.
The fact that people had time to grab a bag is either a symptom of a slow evacuation or some people thought the risk or grabbing their bag was worth it, even if it harmed them and others.