The closure and eventual reduction of the temple was directed by Vespasian in response to sedition in Alexandria that was sparked by zealots who had escaped the Roman counterinsurgency operations in Judaea. This act is decidedly odd – the Romans were far more inclined to co-opt or adopt foreign divinities and sanctuaries than desecrate them. Syncretism of divinities served to incorporate regional cults into a wider, cosmopolitan, and hence Roman, identity. Even when the Romans leveled cities to inspire terror and compel obedience, they were wont to spare sacred structures.
So why was there a change of policy? I would like to propose an explanation which rests on Vespasian’s service in Britain as legate of the legio II Augusta. Resistance to Roman rule in Britain was inspired in part by the druids who fostered a strong identity and a measure of unity. In Roman eyes, the druids were fanatical and antisocial, and they undermined the more constructive Celts. As a result, the Romans developed a policy of targeting religious strongholds, and burning groves and shrines. In the exhausted peace after the Boudica revolt, it may well have seemed to Vespasian that the policy of exterminating the people and destroying the places that gave insurrectionists a tangible and traditional alternative to assimilation had served to overcome the British insurgency, and could be similarly effective against the Jews.