1 Corinthians 1:22-23 KJV For the Jews require a sign, and the Greeks seek after wisdom: [23] But we preach Christ crucified, unto the Jews a stumblingblock, and unto the Greeks foolishness;

Why is Christ a stumblingblock unto the Jew?

The Jews largely do not believe Jesus is the Messiah, they do not believe he is the only begotten Son of God. They believe in their hubris they can approach God the Father without the Son.

The good news: Romans 11:26 KJV And so all Israel shall be saved: as it is written, There shall come out of Sion the Deliverer, and shall turn away ungodliness from Jacob:

I think soon Israel will realize their error and come to Jesus in mass. The only questions left are who will lead them to the truth and when will it occur?

Acts 4:10-12 KJV Be it known unto you all, and to all the people of Israel, that by the name of Jesus Christ of Nazareth, whom ye crucified, whom God raised from the dead, even by him doth this man stand here before you whole. [11] This is the stone which was set at nought of you builders, which is become the head of the corner. [12] Neither is there salvation in any other: for there is none other name under heaven given among men, whereby we must be saved.