One reason Christians don't practice astrology, quite simply, is because the Bible tells them not to. Astrology and other forms of fortune-telling were very common in the ancient world, but the Bible writers scorned them and even called them "detestable practices" (Deuteronomy 18:12).
Why is this? One reason is because God did not create the stars to give us insights into the future, but as a witness to His power and glory. The Bible says, "The heavens declare the glory of God; the skies proclaim the work of his hands" (Psalm 19:1). When you look at the heavens, do they point you to God? Or are you only preoccupied with yourself and your future?
Another reason Christians don't follow astrology is because God has given us everything we need to know about the future in His Word, the Bible. No, it won't tell you what is going to happen to you next week; if it did, would you learn to trust God and seek His guidance? I doubt it. But the Bible does tell us all we need to know, for it tells us that some day Christ will come again.
When He does, Satan will be defeated, and Christ will reign forever and ever. Don't put your hope in something that cannot save you. Instead, put your hope in Christ, for He alone can forgive your sins and take you to Heaven.