Obviously Christianity had a great "selling product" to 1st Century Jews (the Apostles of course, being the first) (And naturally later to "Gentiles"/Romans/etc).
I see it in the same manner as the Protestant Reformation that came 1500 years later. Just as Catholicism had become stagnant, even corrupt....it seems that Judaism had similar problems around "Zero A.D." and thus a message like Jesus' had appeal.
And of course, it also helped replace the other "sacrificing animal guts to a capricious God" religions of the polytheists of Greece and Rome.
And like Catholicism after the Reformation....Judaism "quasi-reformed" itself to insure its survival, still confident that Jews were "God's chosen people...even if Man had failed the faith".
But at its core, Judaism still relied (and still relies today) on the same brutal, tribalistic, even homicidal psychotic named "Jehovah"