Why did Philip preach Jesus using the Old Testament instead of the New Testament?
Philip preached Jesus from the Old Testament because the New Testament had not yet been written. The Gospels, Romans, and other New Testament books did not exist, so the only Scripture available to him was the Old Testament. Starting from the same passage, he proclaimed Jesus to the eunuch.
Les explains that Philip began at the same Scripture and preached Jesus from the Old Testament. The New Testament was not yet written, so Philip could not read Romans, nor could he take the eunuch to Matthew, Mark, Luke, or John, because those books had not been written. The only text Philip could use was the Old Testament. Les also notes that the account does not say Philip preached Jesus crucified and resurrected. Later, the eunuch's request for baptism shows what Philip had been telling him: if he believed Isaiah 53 spoke of Jesus, who had walked three years in Palestine, been put to death, raised from the dead, and God was still ready to let Him be King of Israel. Israel's responsibility under the law was to repent, especially of the sin of crucifixion.