Actually they did. Josephus said, 'the sedition destroyed the city, the Romans destroyed the sedition.' If you read his harrowing account you will discover that the murderous bands of Jews who had taken over the city fought and destroyed each other and brought the city to ruins. It is probable that they too set fire to the Temple to prevent its desecration by the Roman standards. And it was this destruction of the city which led to the cessation of sacrifices in the midst of the seventieth seven. But that doesn't suit what people want it to say. Why spoil a story with mere facts.
And this especially so as the prince is called nagid, which is almost always reference to a Jewish prince. Saul, David and Solomon nd Hezekiah were all called nagid.