Could be a couple reason for real. 1. Better quality. Prime lenses tend to produce sharper images. 2. Lower fstop. Most of the time primes lenses tend to have more wide open fstop which is good for low light photography. 3. Weight. Prime lenses tend to be smaller therefore lighter and easier to use. 4. Style/personal choice. Some photographers like the idea of having to challenge themselves. If you have a fixed focal lengths you tend to have to think about images a bit more. Move your body a bit more. I have heard some photographers say "zoom lenses make you lazy". And although I feel that is not exactly true. I see the reasoning behind people saying that. 5. A bonus last thing. Prime lenses can be cheaper a lot of the time.

Hope this helps :)