Perhaps the better question is why did Rabbi Akiva support him and inspire the masses to join him in the failed rebellion. How could the leading daas Torah not have seen in his ruach HaKodesh how many hundreds of thousands would be killed just in Beitar?
The rebellion was clearly a mixture of religious and political aspirations. He reasoned if Hashem could help the Maccabees defeat the Syrian Greeks he can help the rebels fight off Rome. The previous rebellion failed because of disunity and his rebellion will succeed. He had the leading rabbi on his side so the people flocked to his side.
I have been struggling with this since moving to modern Beitar