Another possibility is that should zoom and fast lenses be affordable , then it will snatch away market from fast primes. E.g. if we have 18 - 135 F2 even if it is small, then people might not want to get primes such as 135F2. Based on my own opinions, ever since the 70 - 200 f2.8 lenses are out and improving, there are lesser and lesser of (canon's) 200mm f2.8 prime being sold. Sometimes in BnS , people are selling at such a low price of $700+ yet there are no takers.
So it's like if you want to have best of both worlds in terms of very fast lenses and versatility of zoom, buy zoom and prime so that company can earn more.