The bulk of the items turned in for recycling as electronic waste are actually plastics. Most computers, televisions, notebooks, and so forth are in large part made up of plastic with just a relatively small mount of metal and other materials.
Many e-waste recycling laws for recyclers do not include those plastics, so the majority of the electronic waste that is recycled (rather than re-used) is still sent to the landfill once the metals have been extracted.
Plastics are not easily recycled for many reasons. Some can be simply melted down and re-cast as new objects or shredded and and incorporated in other materials for different uses. Most plastics, however, are not so simple and require complex handling to recycle, so they are often not recycled, but are instead dumped.