This is actually a fairly complicated topic, but the short answer to your question is that the materials that are involved in static electricity are generally the ones that do not conduct electricity very well (materials that are insulating). Materials that conduct electricity well such as metals, do not maintain the required charge imbalance for very long; this is because the conductivity of the metal allows the charge to not collect on the surface. However, it is possible that metals can retain some static electricity, but it more typically requires a very fast separation. An example of this might be a metal powder or small particles that contact a surface. Hope this helps.