The amount of energy released from a fission reaction is much greater than the energy released from a chemical reaction because in a fission reaction
(1) mass is converted into energy
(2) energy is converted into mass
(3) ionic bonds are broken
(4) covalent bonds are broken

Mass is converted into energy.

Explanation:  There are mainly 2 types of nuclear reactions -

a) Nuclear Fission - in which one heavy nucleus gets break into two or more products releasing tremendous amount of energy especially kinetic energy. The amount of energy released due to mass-energy equivalence principle is higher as some amount of mass of the nucleus gets converted into the energy thus decreasing the mass.

The image of the reaction is attached.

b) Nuclear Fusion - in which 2 or more nuclei fused together to form a heavy nucleus  .