After Aurelian captured Zenobia he heard the Palmyrans had rose and killed the Governor and the Roman force. He returned to Palmyra and destroyed the city and massacred the population.
At least Aurelian didn’t destroy the arch, if it was indeed put up by Septimius Severus.
Maybe Aurelian spared it because it was a symbol of the Roman empire (or republic, as they persisted in calling it).