The main reason is that most Jewish people don't believe that Jesus was the Son of God. They believe that he is yet to come.
There are certain branches of Judaism that do believe that Jesus is the Messiah though, they just still follow Jewish customs.
There are other smaller differences between Judaism and Christianity too. Jewish people worship in synagogues and Christians go to sanctuaries. Jewish people also have diet restrictions.
Something that most people don't know though is that Jesus was Jewish. Christianity didn't just come out of no-where. It didn't form until later on in his life (maybe a little after).
Edit: One other difference is that Jewish people believe that you have to obey the 10 Commandments to go to heaven. Christians believe that you still should follow the 10 Commandments, but that Jesus is the only way to get to heaven.
You have too many Christian answers here who have no idea what Judaism is about. One does not learn Judaism from other Christians nor from the Christian Bible (NT).
Rather than being about individual "salvation," Judaism is about working together as a community to bring holiness into our world. The concept of a messiah is entirely different in Judaism. For Jews, the belief that God would become human is heresy. The messiah will be a great human leader who will usher in universal peace on earth. We seldom mention or think about the messiah, but our mission is to work together to "repair the world," (tikkun olam).
We don't have a place of eternal torment, and believe that everyone has a share in the afterlife regardless of religion or lack thereof.
This book is an easy read and I think you would enjoy it:
"The Complete Idiot's Guide to Understanding Judaism"
by Rabbi Benjamin Blech
This one is another easy read:
“To Life” by Harold Kushner
Jewish ppl say Jesus Christ is a man.
Christan's say Jesus Christ is fully God and fully man.
Muslims say Jesus Christ is a prohet lesser than Alla.
Buddhists & Hindu's say Jesus Christ is a man of peace.