It would certainly be possible for Canon to produce compact wide angle primes in EF-S mount, provided that you were willing to accept some performance compromises. Pentax produce the DA 15mm f/4 ED AL Limited; DA 21mm f/3.2 AL Limited; DA 40mm f/2.8 Limited & DA 70mm f/2.4 Limited as their pancake prime set for APS-C DSLRs. Canon may even have a slight design advantage, if one considerers the original definition and purpose of the EF-S mount (Short
-back focus - i.e. the rear element of the lens can be closer to the image sensor than for a standard EF mount lens; see http://www.the-digital-picture.com/News/News-Post.aspx?News=2544
for more details).
The only reason that I can see for Canon not producing such lenses is as stated by others above; Canon believes that the demographic for APS-C camera users is EF-S zoom lenses for entry level users and the "more pixels on target" users who use the longer EF mount telephoto zooms and primes. Canon seem to think that introducing the new 24mm & 28mm f/2.8 and 40mm f/2.8 alongside the 6D, will encourage the enthusiasts who want wide angle and are still using the 60D - 7D models, to upgrade to full frame. I believe that they have underestimated how many serious enthusiast are out there who are unable or unwilling to spend multiple thousand of (insert currency) to upgrade to full frame. It is no wonder that these people are constantly on forums such as this, complaining about the lack of 60D and 7D replacements and how Canon seems to have forgotten them.
Just in case you think I'm a Canon basher, Nikon are just as bad in this respect (except for the new D7100 -but where's the D400?). Check out Thom Hogan's blog; the lack of wide angle F mount "DX" (i.e. APS-C) primes is a constant complaint; he has responded by replacing his DX kit with an Olympus OM-D EM-5 based system. The downside of this is either requiring two separate systems (expensive), or losing your excellent Canon long lens options and DSLR performance.
Come on Canon, the EF-M 22mm f/2 STM is the one aspect of that system that draws praise; surely an EF-S mount version is not beyond reason? If that is a success, how about an EF-S 15mm f/2.8 as well?