The nuclei Rb83 and Rb85 have the same charge. Rb85 is heavier than Rb85, but gravitation is too weak to take into account. So why orbital electron falls into the nucleus of Rb83? What is the reason?
It is not a matter of "falling in": all s orbitals have non-trivial probability densities at the center.
It is about energy balance in the nucleus.
Kr-83 is a lower energy configuration than Rb-83 by enough to make up for the neutrino and the gamma(s). Evidently Kr-85 is not a sufficiently lower energy state than Rb-85.