Question

why electron can't stay in higher orbit after gaining energy?​

Answer 1

When an electron in an atom has absorbed energy it is said to be in an excited state. An excited atom is unstable and tends to rearrange itself to return to its lowest energy state. When this happens, the electrons lose some or all of the excess energy by emitting light.

Answer 2

Electrons can absorb energy (discrete) to jump from lower energy states to higher. Once they reach higher energy states they become unstable and jump to their ground state by emitting photon (of specific energy).