Why is the carboxyl proton in salicylic acid more acidic than the phenol proton?
More resonance structures does not necessarily mean more stabilization. 

Phenolate has a negative charge at an oxygen atom which cannot be shared without "disturbing" the aromaticity of the ring. This is in part because in the resonance structures drawn, there are negative charged carbons within the aromatic ring, which either implies that:

The ring is still aromatic, so there must be a free electron pair in a p orbital (very high in energy).

Or:

The free electron pair is in an sp³ orbital, which means the ring is no longer aromatic.

In the case of the carboxylic acid, the charge is distributed between two atoms more electronegative than carbon and the ring retains its aromaticity.