Resonance:

We have described already that how resonance can stabilize a negative charge via delocalizing it over two or more atoms. This describes why a  carboxylate ion is much more stable as compared to an alkoxide ion. The negative charge in the former can be delocalized among the two oxygens while the negative charge on the former is localized on the oxygen. We employed this argument to describe why a carboxylic acid is a stronger acid than an alcohol. We can make use of the same argument in reverse to describe the variation in basicities among a carboxylate ion and an alkoxide ion. Because the latter is less stable, it is more reactive and hence is a stronger base.

Figure:  (a) Carboxylate ion; (b) alkoxide ion.