It is true that the convenience and flexibility of the zoom lens are noticeable. However, the complex lens anatomy and its zoom system are big disadvantages. Imagine that the light has to go through many lens layers and the lens layers have to move when the zoom system is active. As a result, the sharpness is the first affected factor. The second obstacle is that with the change of zoom lens system, the open of aperture depends on the focal length and a number of light decreases at a small aperture which can lead to color change.

http://rephotosolution.com/blog/prime-lens/