The nuclei Rb83 and Rb85 have the same charge, but Rb85 is heavier than Rb83. While gravity acts more strongly on Rb85, this is probably not the factor producing the stability of Rb85. So, why does the orbital electron fall into the nucleus of Rb83, and Rb85 is stable?
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.