The airlines make a huge amount of money from freight and from passengers bags. The airlines will not guarantee the safe reunion of passengers with their bags nor take any responsibility for their airline or agents mishandling of the bags. Passengers who have left their bags in aircraft due to an evacuation have to accept that they will be rifled for valuables before they get them back (if they ever do) [cf Southwest wheelbarrow at LGA when a reporter lost his laptop without recompense].
So - passengers know that if they want to keep their valuables safe they have to be kept with them in the cabin because the airline will not keep them safe. By definition then the bags in the cabin normally carry items that the pax consider valuable. The pax are therefore sorely tempted to retrieve these valuable items when told to evacuate as they know they will have to find them on eBay otherwise.
The cure is in the hands of the airlines. They should take responsibility for loss, theft or damage to checked bags _and_ for bags left in an evacuation. The prices for checking bags should not be so usurious (they are seen as a profit item by the beancounters).
Passengers should be told to have - or be given - a body belt or shoulder bag that can contain important papers, money and small valuables. These shoulder bags could be worn during takeoff and landing and would be no hindrance to evacuation.
It is your company's fault that pax take their bags with them - have a quiet word with your management. It may be more effective than fulminating here.