The primary reason is because Jesus of Nazareth did NOT meet their presumptions and assumptions about their Messiah, whom they expected to come in power and great glory and restore Israel as prophesied in Scripture (which He will at His Second Coming). They forgot all about passages such as Isaiah 53 and Psalm 22, and even now refuse to believe that those applied to their true Messiah.
Nobody forgot them. In fact, you can find very early Christian writings where Christians speak of these passages and the Jews of the time say that they believed they didn't apply to the Messiah. It's not some vast conspiracy. It seems that Christians interpreted these passages different from Jews because of their belief the Messiah had come.
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