Just because of band gap. The forbidden energy gap keeps the conduction band at high energy by an amount to equal to band gap energy from the valence band edge. If you compare energies of electrons present in conduction band and valence band, they significantly differ by an amount equal to band gap energy. The low energy electron presents in a valence band requires an energy equal to band gap energy to excite to conduction band. Consequently, the electrons present in conduction band possess high energy compared to electrons present in valence band. At absolute zero K, the low energy states present in valence band are usually completely occupied where as the high energy states present in the conduction band are unoccupied.
