I think it's the sweet spot of two factors, then one economic one.
1. As seen above, wider lenses are more complex with more curvature to front and other elements, more coatings and aberration problems to deal with.
2. The longer the focal length, the narrower the the angle of light it takes in, thus you need a bigger and bigger front glass elements to bring in the light needed for a given aperture. Look at the size of a 400 2.8 compared to a 50 2.8, just like a telescope, it must bigger and bigger to gather that light.
The above 2 "rules" have a sweet spot around 50mm. I doubt 45mm, or 60mm is that different, but 14mm, and 400 are sure as heck way different. And if the true sweetest spot was 48.7mm, that just sounds awkward, while 50mm sounds right.
3. Economics. Canon probably sells 1000 50mm f1.8s for every 1 400mm f2.8, so economy of scale kicks in. Lowering the price of the 50mm dramatically. Actually i recently bought an 18-55mm IS for $88 brand new. That is probably Canons most common lens now.
Anyway there is probably way more "factors" at work, but I think those are the big 3.