The supermarket chains have fewer products to give away because people are paying closer attention to the food they buy. People now buy more of their food at the last minute. More and more food is being sold after its best before date. That's because the cost of food is increasing. That's why there are fewer products making their way to the food banks.
Getting back to Second Harvest, its approach, which is to rescue food throughout the supply chain, makes it a useful model in my opinion.