This type of decay produces a proton, an electron, and an electron antineutrino, when one of the down quarks in a neutron decays into an up quark. This down quark decays into a much less massive up quark by means of a virtual discharge known as a W-minus boson. The virtual discharge, as in proton decay, accounts for the new particles present after the decay. There is no special attribute of protons that would inhibit their decay. Proton decay would not violate any of the conservation laws by which all other particles are allowed to decay.
Some physicists think they are.
To justify this premise a new conservation law was created. It is called the Conservation of Baryon Number. Since protons are the least-massive baryons they cannot, by this reasoning, emit energy and become a less massive baryon like other baryons. I fail to see the merit in a law conceived simply because the decay has not yet been observed. People are making attempts to measure this event.