Why doesn't a Planck type derivation work for Hawking radiation?

In Planck derivation of black body radiation. Planck hypothesized $E=hnu$ and got the correct black body radiation spectral function.

What is confusing me is why doesn’t similar arguments work for Hawking radiation? Hypothesizing discrete spectrum of some physical quantity, maybe area of event horizon of order of Planck scale and then using it to derive the hawking radiation.

Essentially above reasoning implies the fluctuation in the hypothesized discrete quantity leads to creation of quanta of Hawking radiation. In other words this quantity is hypothesized as microstate. Has this approach addressed in the literature? If so what are it’s shortcomings?