Lets consider semi conductors, if the valence band is only partially filled, then some electron probaly gain some Kinetic energy from thermal excitement. This increase in energy of the system can only result in electrons in crossing the energy gap and into a high quantum state. If the same scenario occured to an insulator, the energy gap is large and prevents electrons to be promoted to a higher quantum state. And since electrons can only remain in these quantum states, it must remain in the valence band, keeping them filled.
