Why did classical physics fail to explain blackbody radiation at high frequencies?

In classical physics, the failure to explain blackbody radiation at high frequencies—a discrepancy known as the ultraviolet catastrophe—stemmed from the Rayleigh-Jeans law.

This classical model relied on the equipartition of energy, which assumed that all frequency modes of electromagnetic radiation could radiate with equal probability. Because the number of these frequency modes increases without limit as frequency rises, the classical formula predicted that the intensity of emitted radiation would also increase infinitely at higher frequencies. This contradicted experimental observations, which showed that radiative power eventually falls at high frequencies.12

Max Planck resolved this by proposing that energy is not continuous but is instead absorbed and emitted in discrete "chunks" or quanta, defined by the equation $E=h\nu$. By introducing Planck’s constant ($h$), he established that at a given temperature, there is insufficient thermal energy to frequently excite the high-energy quanta required for high-frequency radiation, thereby preventing the infinite energy output predicted by classical theory.21