“why didn’t he just prevent the Romans from destroying Judea in the first place”
Well, he was also mad at the Jewish kingdom. And while (as do all things), the Roman sack fit His plans, His promise to punish people who harm Israel stands.
That said, the eruption seems a bit distant.
I would note that Nero attempted to put up a pagan statue of himself on the Temple Mount and was promptly struck dead before the effort could be completed.
That seems pretty direct.
Correction: Caligulia.