How did quantum theory explain why electrons do not fall into the nucleus?

In the Rutherford model, the atom was considered unstable because classical electromagnetic theory predicted that an orbiting, charged electron would continuously emit radiation, lose energy, and spiral into the nucleus.123

Quantum theory, as first applied to this problem by Niels Bohr in 1913, resolved this by postulating that electrons can only exist in specific, stable "allowed" orbits with fixed energy levels. In this model, an electron cannot lose energy continuously; it can only transition between these discrete states by absorbing or emitting energy in fixed quanta. The atom remains stable in its lowest energy state (the smallest orbit) because there is no lower energy level available for the electron to occupy.4132

Later developments in quantum mechanics further refined this by describing electrons using wave functions known as orbitals, where the electron's position is defined by probability rather than a simple classical path.42