This is the case because in a fission reaction you are essentially having a particle or neutron split apart and or interact with an unstable nuclei, causing smaller nuclei to form upon the collision, which would most likely cause a chain reaction to occur, since a great deal of energy is involved in breaking apart the nuclei, more energy would be released if this is done, compared to electrons being transferred to form chemical bonds, or causing electrons to be dislodged from a chemical covalent bond.
