This, this is might be why. The OGL is spacial in the world of open licenses in that it allows the creator to split content into "open content" and closed content ("product identity" in OGL speak). CC-BY does not do this.
Remember how everyone was upset with WotC adding language to 1.1 that they could use only OGL content, well now they can do that just as easily if you go CC. You can only seperate your contnet if you do an SRD for your open content in CC-BY, and publish your actual commercial stuff without CC-BY. That is a lot of work and messy.