My childhood rabbi (and this was about 60 years ago), when asked by a student in Hebrew school why Jesus is not the Messiah, said that the Messiah is supposed to bring peace on his first and only visit to the world and Jesus did not do so. (Where did the notion of a Second Coming of the Messiah come from?) This, I think, is as good a reason as any. For me, personally, however, maybe if Jesus had not claimed to be Gd, I would have believed He was the Messiah. But a fully 100% man who is also 100% Gd is, to me, the most difficult thing to believe. Then there is also the changing or fulfilling of the Law, which is supposed to be eternal and perfect just the way it is. And the prophetic warning by Moses in Deuteronomy not to believe a miracle-worker or dreamer who may lead the Jewish people to belief in another god or gods. The whole elaboration of hell in the New Testament is yet another problem I have, and the idea that we are born in (original) sin and, at the same time, in the likeness and image of Gd. So many other things as well.