I was just interest why it happened that at the foundation of Christianity lies the Old Testament? It was just most authoritative religion 2000 years ago? Or any another reason?
I was just interest why it happened that at the foundation of Christianity lies the Old Testament?
Christianity began as a split within Judaism, so that seems fairly logical.
It was just most authoritative religion 2000 years ago?
No, far from it. Judaism was one of the many religions in the region. In the area of the bible countries (now: Israel, Syria, Egypt etc.) there were many more religions, but Judaism was locally the most important religion.
In some areas (Jerusalem, Galilee) it was the most dominant religion, in other areas it was not. The whole area was a patchwork of very different religions. Samaritans didn't get along with the Jews, whole areas were completely Greek. Greeks and Jews were often at odds. As far as I know Baal was worshiped in several places.
Not sure if this wouldn't belong more on another site. But one thing to consider is that Judaism is not a very proselytizing religion - it's tough to convert to and is really aimed at people of Jewish ancestry.
On the other hand, Christianity aims to convert people to it. So, rather than "just" a question of doctrine, as you can see with various branches of Islam and Christianity evolving, the notion of appealing to non-Jews would just not fit in with core Judaism.