The primary reason is that Canon would not want to lock quality-conscious photographers into APS-C systems - when you finally start thinking about full frame, you'd have to ditch all the lenses you've collected, and might as well choose some the rivaling system (where, by the way, is similar discrimination against APS-C).

Another reason is that for wide to normal angle photography, APS-C is at disadvantage due to the crop factor. You need glass with shorter focal length (more expensive) and have less control over depth of field - dictated by smaller physical aperture at same angle of view from on the shallow side and by reaching diffraction sooner on the higher end. So, asking for high quality at reasonable price, APS-C only makes sense for longer focal lengths, where the crop factor gives extra reach and extra depth of field (for macro). But as Matt Grum pointed out, in longer focal lengths the differences wouldn't be remarkable enough to justify designing a lens exclusively for APS-C.

Sigma marks their APS-C lenses with "DC", and has some primes to offer - a couple of f/2.8 fish-eyes and a 30mm f/1.4. In tele zooms, they're offering 50-150 f/2.8.

Tamron distinguishes exclusively APS-C lenses as Di II. There's one prime, 60mm f/2 Macro. There's also one tele zoom, 55-200 f/4-5.6.

Tokina's line for cropped sensor is DX. You'll find 35mm f/2.8 Macro prime, and no tele zooms (50-135 f/2.8 was discontinued in 2009).

For Canon and Nikon, full frame is the primary market. People who buy low-end bodies are not very likely to start hoarding high-end optics; and if they want, they can use full-frame lenses. If you're into building a system based on APS-C and primes, Pentax is a more suitable platform for you.