I'd guess that the logistics of getting a relatively small number of items to a central distribution point must be very difficult.  Shops in an area tend to close at similar times so it would take almost one volunteer for each shop to collect goods and with unwrapped items special containers would be necessary and perishable items would have to be consumed in a short time frame.

It's a matter of resources, most charities are desperate for volunteers - we can all play our part by getting involved.