Particles can only decay into lighter particles because energy is conserved. Remember, mass is a form of energy (E=mc^2). If a particle at rest decays the total mass of the decay products must be less than the mass of the original particle. The leftover energy becomes kinetic energy of the particles.
Ditto everything that Arttu said! The key thing to remember here is that energy is always conserved. It cannot be created or destroyed, but can change into different forms, remember that it will all start to make sense. They can’t decay into heavier particle because then where would that extra energy come from? it can’t just be made out of nothing, it has to come from somewhere (remember that mass can also be thought of as a form of energy) ”Left over” mass can either become kinetic energy of the lighter particle or be emitted as a photon of light (light can be thought of as a particle OR a wave).