Because by definition, it's impossible for the Old Testament to be Christian. To be pedantic, Christian, by definition, is "Christ Follower". If Christ had not been born on this earth yet, it is impossible for anyone to have been following Christ. Ergo, the Old Testament can't necessarily be "Christian". That is not, however, to say that we as Christians do not believe in the Old Testament. We see it as a quasi-historical record, with much that can be learned from that history. However the Christian faith really can't be defined as such until after Christ begins his ministry.
In a very narrow and pedantic sense, that may be true, but...
Christians, by definition, believe that Christ is One with God the Father and the God the Holy Spirit, all three of Them existing eternally, uncreated, in one Godhead. Those who worshiped the Jewish God in Old Testament times were therefore following Christ, whether they understood that or not. Abraham's faith in God was credited to him as righteousness (the central idea of Christianity). As Ron points out, Christians understand that the Old Testament prophecies spoke of Christ, and that many appearances of angels may have actually been pre-incarnation appearances on Earth of Christ himself.
Following the Apostle Paul, I would have to go further than Ron, and say that Christianity is not just "a Jewish sect;" it is true Judaism. (Galatians 4)
