I hope that, at some point, my parents will return to a life of semi-normalcy and be able to contribute something here, because they probably know more about this than me. My impression is that there were lots of changes happening in beliefs about the afterlife around the time of Jesus, what with the Pharisees and Sadducees duking it out over resurrection. But you know that stuff. I guess the interesting question is, why was it changing? And why did people hear this "hell" idea from the Zoroastrians and go, "That sounds like a great idea to me!"

I'm pretty sure I've sent you these web pages before, but I find them handy (if tragically designed):
Teachings in the Hebrew Scriptures about the Afterlife: A Liberal Interpretation
and the bizarrely un-parallel-ly named
Liberal Interpretation of Passages from the Christian Scriptures on the Afterlife: 1st Century CE

But of course that doesn't answer the why question.