Why not the Old Testament?
Like I said, when Christ died on the cross, the Old Covenant for God's chosen people (the Jews) died with him. He established a New Convenant for both Jews & Gentiles. That covenant is described by the inspired writers of the New Testament. The church was established in the New Testament, so it seems the best place to me to find out how the church is supposed to act. We don't ignore the Old Testament, but we certainly don't follow the old law, for example, sacrificing animals for forgiveness of sins, keeping the Jewish festivals, or worshipping on the Sabbath (Saturday).
"Why not the Old Testament?"
Same reason we do not follow the Koran, and aren't you glad?