What is the reason why protons and electrons do not collide?
They do collide.  But it's not like cars crashing and smashing each other up on the highway.  Their quantum mechanical wavefunctions overlap and the electrons do in face settle in with wavefunctions that are centered on the nucleus.  But the low mass of the electrons, the Heisenberg uncertainty principle (which has its greatest effect on low-mass particles), and the Pauli exclusion principle (electrons can't all pile up in the most stable quantum state)  cause the electron wavefunctions to get diffused out around the nucleus.  Instead of getting smashed up, the electrons basically surround snd embed the atomic nucleus.