I've always wondered why fiberglass bicycle frames were not made. I'm cautious with saying "never," as there might be the occasional rare one out there. Yet I am close to using the word "never" with regards to seeing a fiberglass bicycle because I have never seen one marketed or in real life.
Fiberglass is a lot cheaper to make than carbon fiber, and it has good strength to weight ratio. Furthermore, it is more shock-resistant than carbon fiber because of its lack of stiffness, which I think may also act as a drawback. Is adequate stiffness really that important?