 Solution

Propionic acid is a weak, relatively high-boiling acid: its normal boiling point is about $141^{\circ }\mathrm{C}$, compared with about $118^{\circ }\mathrm{C}$ for acetic acid. It provides mild proton catalysis while remaining in the hot orthoester mixture more effectively. This is useful because ketene-acetal formation and the thermal Johnson-Claisen rearrangement require heating, whereas strong acid would threaten the pyran acetal.

This is a practical catalyst choice, not an absolute mechanistic prohibition on acetic acid; acetic-acid-catalysed orthoester rearrangements are also possible.