Because it does not fulfill the Torah's criteria.
Just what IS (are?) the criteria?
Thank you for asking.
First of all, the messiah isn't the heart of Judaism or of the true religion for gentiles. The Torah is the is the "logos" from which the world was created and the purpose of existence. The messiah is merely an expediter--the ultimate expediter, true, but an expediter nonetheless--of Torah observance.
The Mashiach must be descended from David through Solomon (not any of David's other children) through the male line (which one born of a virgin couldn't do). He will be a literal Davidic King, defeat `Amaleq and all Israel's enemies in battle, regather the Jews to Israel, rebuild the Holy Temple and restore the Temple Service, and inaugurate a period of literal (not "spiritual") peace. All false religions and false "gxds" will be disappear and the entire human race will acknowledge and worship the One True G-d.
Now, I am aware that in the past two or three centuries some chr*stians have developed the idea that J*sus will fulfill these prophecies at some time in the future. This is not what chr*stianity originally taught (originally it taught that all these prophecies were fulfilled at once in a "spiritual," non-literal manner). This means that if this is so, classical chr*stianity isn't the true religion at all. Furthermore, until J*sus fulfills these prophecies he cannot be the messiah. Furthermore, until they are all fulfilled Mashiach has not come.
Here's a web site with information on the true concept of Mashiach: Messiah and Tanach. Please note the unfortunate use of the word "fundie" further down on the page. As I am a Fundamentalist myself I resent the use of the term in this derogatory way. Unfortunately, even fundamentalists of other religions think that only a certain kind of American Protestant is a "fundie." It's stupid and short-sighted of the author of the page, but it doesn't nullify the theological arguments he marshals.
MUST be?
Luke 2:25-32
[25] Now there was a man in Jerusalem called Simeon, who was righteous and devout. He was waiting for the consolation of Israel, and the Holy Spirit was upon him.
[26] It had been revealed to him by the Holy Spirit that he would not die before he had seen the Lord's Christ.
[27] Moved by the Spirit, he went into the temple courts. When the parents brought in the child Jesus to do for him what the custom of the Law required,
[28] Simeon took him in his arms and praised God, saying:
[29] "Sovereign Lord, as you have promised, you now dismiss your servant in peace.
[30] For my eyes have seen your salvation,
[31] which you have prepared in the sight of all people,
[32] a light for revelation to the Gentiles and for glory to your people Israel."
Is this what Simeon was talking about?