The reason is: Because it can. Also known as "Everything that can happen, will happen.". We already know from statistical mechanics that the lowest energy state is the most likely. In QM, the fact that all things drop into their lowest allowed energy state is even simpler: As long as there is a non-zero transition amplitude $e^- \to e^- + \gamma$ (where the $e^-$ has dropped into a lower energy level and radiated the energy difference as a photon), this transition will, sooner or later, happen. Since for an electron that has been raised into an excited state by hitting it with a photon the reverse process has certainly non-zero amplitude, the electron will, sooner or later, drop to the lower energy level. Once there, is has no chance to go anywhere without energy coming from the outside, so it stays there.