Why hasn't Canon made a wide angle lens with IS?
Regarding IS and wide angle lenses, let's take a step back and remember what the problem is that IS is a solution to. Unwanted camera/lens shake/movement during exposure causes the light from a given point to be displaced across the sensor (or film) resulting in streaks or blur in the recorded image. The apparent displacement for a given amount of shake/movement is greater with longer focal lengths. Consider an object that is 1 degree of the FOV. That object will not only occupy a much larger portion of the sensor with a lens that will record a 10 degree FOV, but any unwanted motion will also be larger and more objectionable than with a wide angle lens with a 100 degree FOV and the same object.
The "classic" solutions to this problem are normally: a) to steady the camera/lens with a tripod, monopod, bean bag, gyroscopic device, etc.; b) use a faster shutter speed so that the extent of the motion recorded during exposure will be less objectionable.
In 1976 Canon patented Image Stabilization and introduced the first interchangeable SLR lens to use it in September 1995, the EF 75-300mm f/4-5.6 IS USM, followed in March 1997 with the EF 300mm f/4L IS USM and EF 28-135mm f/3.5-5.6 IS USM in February 1998. The latter being the first EF lens with IS in the wide angle range. The first IS version only provided about 2 stops of improvement and had issues when on a tripod or panning with a subject. Later IS versions have improved substantially.
Canon's IS works by shifting a lens group to compensate for camera/lens shake/movement and reduce any image displacement at the sensor that would have resulted. This benefits focusing and composition as well as the recorded image.
The current Canon lenses with wide angle coverage and IS are:
EF zooms: EF 28-135mm f/3.5-5.6 IS USM (2/1998), EF 28-300mm f/3.5-5.6L IS USM (6/2004), and EF 24-105mm f/4L IS USM (10/2005)
EF-S zooms: All the current "standard" zooms have IS with wide coverage starting in the 15-18mm (24-29 FFE) range.
The only EF-S prime is the 60mm f/2.8 USM macro which lacks IS.
The shortest EF prime with IS is the 100mm f/2.8L IS USM with Hybrid IS (10/2009) which compensates for both angular and shift shake.
Currently the shortest EF prime with "regular" IS is the EF 200mm f/2L IS USM (4/2008).
Canon has announced EF 24 and 28mm f/2.8 IS USM lenses expected to be available in June. They seem to be anomalous with respect to the rest of Canon's line, and not just for price-point and cost/benefit reasons. With the reduced impact of camera/lens shake with wide angle lenses, the IS benefits would not seem very compelling for shooting stills. As others have mentioned, their IS would be more compelling for video shooters because not only would each frame be improved (intra-frame shake), but there would be less apparent shake between frames (inter-frame shake) which would provide a smoother video. A contrarian would, of course, observe that none of Canon's EF mount cine lenses (2 zoom and 24, 50 and 85mm primes) have IS. Of course at the cine lens price points, Canon may figure users would have other apparatus to stabilize the camera/lens? Maybe the announced 24 and 28mm with IS are targeted at amateur and small under-capitalized production company videographers? It will be interesting to see how well the IS in these new lenses handles panning with moving video subjects (eg kids), which are not always uni-directional.
In short, Canon has had "Wide IS" for the past 14 years, but not extensively and only in zooms (which also had coverage into the telephoto range), probably due to minimal benefit to still photographers. However, with the popular advent of DSLR video, we can expect to see more "Wide IS", including primes.
Thanks for your kindness in reading such a long post.