Because that's a lower energy state. The deuteron binding energy is 2.2 MeV, i.e. the mass of a deuteron is 2.2 MeV below the sum of the masses of a proton + ne

Because that's a lower energy state. The deuteron binding energy is 2.2 MeV, i.e. the mass of a deuteron is 2.2 MeV below the sum of the masses of a proton + neutron. If the neutron inside would decay into a proton, you would have a diproton (^2He). Very unstable, essentially no binding energy, so it's mass is very close to 2 protons, which is still above the deuteron mass. So the neutron can not decay spontaneously. There is not enough energy available.