I'd say the full frame sensor is reasonably well covered at the wide range.
Off the top of my head, there's a Canon and Samyang 14/2.8's, a Zeiss 15/2.8, a Canon 15/2.8 fish eye, the Zuiko OM 16/3.5 fish eye, Canon TS-E 17/4L, Zeiss 18/3.5, Zuiko OM 18/3.5. Several of these are out of production but they are still available in the used market.
You should also remember the TS-E 17 can also work like a 10.8mm or 12mm (depending on how you orient the shift.)
On crops - I suspect the reason is that zoom lenses are quite adequate and it's difficult to make a prime significantly faster than the available zooms.
Edit - on re-reading the question
Why are there no less expensive options? I'd venture that the cost of manufacture of most lenses with a given optical quality is the same - give or take 20%. Of course, this doesn't apply to lenses with huge optical elements - optical glass is very expensive - but design, prototyping, manufacture, assembly, test, distribution and retail all requires the same infrastructure.
The cynic in me might also comment that there's less money to be made selling cheap items that are "good enough."