Fruits like raspberries and blue berries mold quickly because they are good surfaces for mold to grown on. Mold is a fungus with spores that disperse through the air and will land on uncovered surfaces such as a raspberry. If the surface is wet, perhaps because the berries get damaged in the store so the skin of the fruit breaks, exposing the juices, it makes an easy surface for the mold to grow on quickly. The berries are also fairly thin skinned so mold that does establish on them can quickly break through the skin and get to the fruit juices that are great for mold to live off of. Refrigeration can slow the mold down because most fungi will grow slower in cooler conditions. The fruit would also be less likely to get moldy to begin with if you limit its exposure to the open air, which is where the spores are floating around waiting to land in suitable growing conditions, if you cover then.