Probably because they recently introduced the EF24-70/2.8L II USM at a price of $2299 and maybe they didn't think the market could stand a higher price for an IS version.
Why the EF-70/2.8L II wasn't IS in the first place and why it costs so much for a non-IS version, I have no idea. Seems to me to be mistake, but they presumably know their market (though I'll admit that I'm starting to doubt that....).
The 24-70/4L IS USM falls in nicely with the 70-200/4L IS USM as a smaller and light set of lenses for full frame, with high image quality and stabilization. There's also the 17-40/L USM so there's clearly a product line that's f4 for size and affordability.
Of course if you want an image stabilized 24-70/2.8 lens there's the Tamron SP 24-70mm Di VC USD
, which is not only f2.8 but is also cheaper than the Canon 24-70/4L IS USM will be.
This is just another hard to understand move by Canon, along with why the 400/5.6L still doesn't have IS and why their initial move into the mirrorless camera market is so underwhelming. The EOS-M is a nice, simple, entry level camera, but feature for feature really can't compete with offerings from Olympus and Sony, especially for the more advanced photographer.