From a "Physics" point of view I would say because of radiation of an accelerated particle:
In Rutherford atom electrons (negatively charged) orbit the positive nucleus, like in a planetary system, obeying Coulomb's Law as planets obey Newton's Gravitation Law, BUT....
Rotating around the nucleus electrons experience CENTRIPETAL ACCELERATION and so should radiate energy as EM radiation (exactly as oscillating electrons in an antenna) and quickly lose energy and spiral into the nucleus...but this doesn't happens!
It must exist some "weird" different configuration allowing the electrons to orbit WITHOUT emitting energy (see good'ole Bohr!!!).