Why can't X item be recycled?
Recycling is a business. Any item has the potential to be recycled, but there must be a market demand. If items are cheaper to send to the landfill than sell as a baled product, then recycling centers won’t accept those items, because it is not financially viable.
An example is Styrofoam or polystyrene. There are few centers that can accept polystyrene. Polystyrene is lightweight, but takes up a lot of space. It isn’t viable for recycling centers to collect and haul products that take up a lot of space, but don’t generate much of a product to sell.
Another example is plastic bags. MRFs do not accept plastic bags in their recycling because they can get stuck in the sorting machines. When bags get stuck in sorting machines, it can cause the whole Materials Recovery Facility to stop, until fixed. Plastic bags can usually be recycled at grocery stores that provide a drop off bin to recycle bags.