Why the transition of electron from an excited state to metastable state in a lasing medium is radiation less?

It depends, there are different decay processes happening in a medium. Not all the decays are radiative (i.e., with emission of a photon). Moreover, regarding photonic channel decay, non-induced transition provoke isotropic photon emission. In lasing you care about spontaneous emission over the cavity direction. Being the fraction of spontaneous emitted photons over the cavity direction negligible, and also considering non resonant photons (usually the transition to metastable states is done with non-resonant photons with the cavity...), these decays do not produce lasing effects.