But the reason is obvious. Instead of quoting from the Hebrew Scriptures which the Athenians could not relate to, Paul quotes from two Greek philosophers. The Greeks could relate to both of these quotes, but they would not have been influenced by quotes from the Bible. It is only later after they joined with Paul that he would expound to them the scriptures. Don't get me wrong.
Biblical teaching is important.
People don't understand what we are talking about. I once was ministering to a friend who did not believe in God. Quoting the Bible to him would have done no good. He didn't believe in the Bible. But sharing with him how God cares about him helped him to be receptive to God's message.