How did you come to the conclusion that furan should have a higher resonance energy? And what precisely do you mean by "resonance energy"? The aromatic stabilization energy?
I think that the best explanation is that the aromaticity decrease as the electronegativity of the heteroatom increases (O is more electronegative than S). A more electronegative atom is less "willing" to donate its electrons to the aromatic system. In effect, the electrons are more localized on a very electronegative atom than in the rest of the aromatic ring. Benzene is the most stable simple aromatic because all of it's bonds and atoms are the same.