This is why Jews are Jews and not Christians, among many other reasons, but I don't think I ever really learned the historical reasoning for the rejection at the time of his death. Whether you believe Jesus is the son of God, he was a real person.
I just don't understand how and why exactly they didn't believe he was the son of God, and how they managed to survive for so long not believing it.
You could ask them but they're in hell.
Personally I think it's because of the same sins they practice today, that being that they think God is a genie and that religion is transactional. That's why they worship the devil these days. They don't want to be good and do good because it's worthwhile in itself, they want to cut the chicken's throat and do a dance and chant the magic words and make stuff happen like the stone age barbarians they are at heart.
Christ's birth and resurrection was in effect God rejecting their slot machine behavior and incessant idolatry.