One reason: It was simply an architectural style that happens to inspire us to look heavenward.
These early church architects designed grand cathedrals and churches that had intricate, soaring steeples. The vertical lines of the steeple helped to visually enhance the lines of the church, directing the viewers' eyes vertically to the heavens. Obviously, this verticality complements part of the mission of the church, to keep us in a heavenly frame of mind, but from an architectural standpoint, this vertical lift gives the architecture a more graceful and pleasing look. The shorter the building, the more squat the appearance; the taller the building, the more graceful it becomes. The early church believed that the church could communicate the truth of the Bible in pictures and symbols to those who were illiterate, such as using the picture in the stained glass to tell stories, as well as the steeple, which helped by pointing upwards devotedly to Heaven. Therefore, the steeple has a dual role in that it helps the congregant in his or her spiritual mindset, and the steeple also helps the architect with a design feature that enhances the overall harmony of the architecture. This is from an article, The History of Church Steeples,