mike, I would suspect that it's because people haven't been clamoring for a 24-70 L IS until fairly recently ... and it's still not clear to me if there's actually a lot of people out there clamoring for it, or if it's just on the CR forums. the 24-70 L is one of the first lenses that anyone who goes full-frame gets, so basically everyone has to decide to upgrade to a 24-70 L IS that costs in the $2K range
I would think that there are three things that determine the timetable for development and release of such a lens:
1. market demand
2. current projects competing for development resources
3. profit goals for original lens to be replaced
unless there is massive and sustained demand for it, canon will probably take its time clearing out the projects it has already planned off its plate before putting a redesign of the 24-70 into the queue.