DISCLAIMER: I understand the reasons for fast aperture lenses and their benefits. I play -to a certain extent- devils advocate.
I think the problem is that for many users the zooms are actually good enough these days, and in particular, the consumer zooms are damn fine for the money. Why do you need f2.8 or faster? Just up the ISO? Etc.
Now you know, and I know, that theres a bit more to it than that, but the 24 year old EF primes that have just been replaced were often just FD designs with a remount and AF, fine for film, not fine for that curse of the modern camera -anal pixel peepers.
To be fair Canon aren't going to sell that many prime 24s and 28s, so there is limited spread of R&D cost, but I cant help thinking they would sell 3 times as many at half the price. Shame as the 28mm 2.8 non-is is one of my favourite lenses.