Because the crystals do not grow equally fast at all sides. They grow at specific sides where the molecules can easily fit in the regular pattern in which the molecules are arranged in the crystal. On other faces, the molecules can grow but much more slowly (they have to be distorted) or not at all (they don't fit). This causes the crystal not be spherical, but having a specific shape: hexagonal (8 growing faces), cubic (6 growing faces) or needle-shaped (2 growing faces) or other shapes.