In the synthesis of aspirin, why is an acid catalyst preferred over a base catalyst?

Salicylic acid is acidic enough to protonate the pyridine and that would not be helpful. By adding an acid, you are ensuring the material is protonated. The use of acetic anhydride generates a base (acetate) and that picks up the proton lost from the acylation of the alcohol. The H2SO4 (or H3PO4) again really only ensures that the salicylic acid remains protonated.