nMass of proton is chosen for
stability
nMass of free neutron is slightly heavier than that of a proton, and it decays to a proton, electron, and an antineutrino with a half life of 12 minutes. Free neutrons cannot persist in nature.
nIf the mass of a proton were increased by just 0.2 %, it would decay into a neutron, a positron and a neutrino.
This decay does not occur - if it did
hydrogen could not exist. Hydrogen
is the dominant element of the
universe. Without it, the universe could not exist