Why was the Jewish temple in jersualam destroied?
At that time, there was a faction in Jerusalem called the
Zealots (the Biryonim mentioned in the Talmud). They, against the
advice of the Torah Sages, instigated a guerrilla campaign against
the Romans in an attempt to drive them from the Holy Land, since
the Roman rule was insufferably oppressive.
The worst crime in the Roman lexicon was revolt and
insubordination; and they responded to the Zealots' provocations
accordingly. They dealt with them with the utmost harshness because
they (the Romans) feared that if they didn't do so, other conquered
areas might revolt also.
Because they couldn't always tell the Zealots apart from the
non-combative people, the Romans simply destroyed the entire city
including the Temple.