A. Canon has an EF 24-70mm f/4 and - as noted above - equivalent focal range EF-S 17-55mm f/2.8 with IS. This means Canon thinks IS on that focal range useful, possibly for some photographers' need, definitely it's bottom line.
B. Canon has four 70-200mm lenses, 2 w/ IS & w/o, which means both Canon and it's customers find it useful to have a choice. In this case, the differentiating factor is price. Why not do the same with 24-70mm lenses, with the differentiating factor being IQ?
As Tamron is already selling a 24-70mm f/2.8 vc, stealing sales Canon could make by releasing a similar lens, my guess is Canon thinks that between it's f/4 IS, f/2.8 IS-less, and Tamron f/2.8 vc, it can't make a profit from turning it's prototype into a selling lens.
That could be due to characteristics of the lens (say price/performance point too low to compete with the Tamron), lack of available manufacturing resources, or whatever, but if Canon was confident it could release one soon, I would expect a development announcement to keep some photographers from buying the Tamron. Not seeing such announcement, I don't expect to see an EF 24-70mm f/2.8 with IS anytime soon.