When conducting experiments to determine the relationship, if any, between light intensity, light frequency, and the development of current from a photosensitive material, experimenters, like Einstein, found that I ~ F, the current was proportional to the flight frequency, not the intensity.
So Al and others reasoned that I ~ F because the light frequency was causing electrons to pop off the material's atoms at discrete intervals commensurate with dE = En - En-1 the energy levels between two different shells, n and n-1, around the nucleus. So they reasoned that dE = h dF, where h is a constant of proportionality and, ta da, dE/dF = h the slope of the curve. Note it's h, not h/2pi.
And what they discovered, and Al published in his Nobel winning piece on photo-electricity, was that h always had the same value, no matter what F frequencies were used. Someone had to give it a name as it was now a constant of nature. So they named it Planck's Constant.