Indeed, 1. is a mystery. But, 2. is a consequence of 2nd law of thermo. Specifically, the lighter the mass of a decay product, the larger the number of possible final states in the kinematical phase space (in general). For instance, if there is just enough energy to make a pair of particles, then only one final state is possible: particles sitting at rest. On the other hand if energy is “left over” in addition to rest energy, then there are many possible final states corresponding to the pair flying outward in different directions. Hence, the decay to the lighter products are entropically preferred. This is mathematically encapsulated in Fermi’s Golden Rule.