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.
When people say that anti-semitism is the "oldest hatred", they are wrong. First of all, because it originates in about 1800, secondly, because even anti-Judaism isn't the oldest hatred or even close to.
All the people who were hated more haven't lived to tell the tale. Have you heard of cathars or Waldensians? Where are the Edomites or Moabites? Where are the Avars? Where are the ancient pagans or the druids? Persecuted out of existence. If there were actual Druids still, as opposed to larpers, they might well claim to be more hated than the Jews... but since they were extirpated or otherwise disappeared, they won't. Doesn't mean we should believe in understandable Jewish delusions which are bad for them and bad for us.
Judaism survived because of Christianity, not in spite of it. According to Christian thinkers like Augustine, Jews were entitled to a certain amount of respect and also to witnesses to the truth of the Old Testament - which has value in that it contains the prophecies of Christ's coming. Claims that Christians persecuted Jews are lies - pogroms, persecutions, etc. were almost never officially sanctioned by either church or state, and the Spanish Inquisition had no jurisdiction over Jews (except in the infinitesimal number of cases where Jews were accused of spreading heresy among Christians), only over former Jews who had converted to Christianity and who were alleged to be Judaizing like so many genitally disfigured, Israel-shilling American "Christians" today actually do.
As for why Jews reject Jesus Christ: they believe that the Messiah is supposed to be a powerful figure who vanquishes all, not a powerless, crucified criminal (in their view).