Why isn't proton radiation by decay on earth known in nature? Perhaps asking for why isn't appropriate in physics, but as there is neutron and alfa radiation what causes proton radiation not to be very common in nature (in laboratory it is seen although)
 A: The closest thing to an answer would be, that if a neutron in a nucleus decays into a proton and an electron (beta radiation), the ligher electron is the one that can escape the nucleus. The proton is heavier, and also strongly bound by the strong nuclear force, so it stays there.
The same goes for neutrons - they aren't emitted in simple nuclear decays, that only happens when nuclei break apart (fission), and it's always neutrons, because heavy nuclei have excess of neutrons that have to go somewhere. Why heavy nuclei have more neutrons than protons? To "dilute" the otherwise overwhelming electrostatic repulsion between charged protons.
A: The strong nuclear decay process emits alpha particles that are just helium-4 nuclei. The weak nuclear decay process emits beta radiation that is an electron (usually) with the internal to the nucleus the process $n~\rightarrow$ $p~+~e~+~\nu_e$, or in some cases if energetically possible $p~\rightarrow$ $n~+~e^+~+~\bar\nu_e$. The electromagnetic process is a rearrangement of protons or charges that emits a $\gamma$-ray photon. 
Neutrons are not emitted in strong nuclear decay, but in fission. These neutrons released from fission initiate further fission.
A classic interaction is $p~+~n~\rightarrow~p~+~n~+~\pi^0~\rightarrow~p~+~n$.  The $\pi^0$ is $d,~\bar d$, and the generation of $\bar d$ with opposite color charge is accompanied with the creation of $d$. and the $\bar d$ in the $\pi^0$ annihilate a $d$ in the other proton, and a net gauge force occurs. this can occur with two protons $(uud)$ or neutrons $(udd)$ and between them. The $\pi^0$ can be $u\bar u$ as well, which is a lighter quark. The Heisenberg uncertainty principle then gives a physical sense for why fluctuations involving the $u$ quark are more prevalent. Since the proton is made of two $u$ quarks this is a more likely process. With proton neutron binding the two forms of the $\pi^0$ are equally probable. Because the $u$ quark is lighter in a complex nucleus the protons are more tightly bound on average. This is even though they electrically repel. 
There are complexities here, such as clearly there is no stable bound state of two protons. Yet in general with a fission event neutrons are emitted, not so much protons. Of course a nucleus subjected to a very high energy scattering particle can spallate into a shower of protons and neutrons. With the heavy ion work at RHIC and LHC that is the final state of high energy collisions between heavy nucleons. 
