I remember hearing that there's two different series he wanted to start. One was sci-fi, and the other steam punk. I think (but I'm not certain) the sci-fi pitch wasn't polished, but people reacted well to the steam punk idea. So he went with that one.
I don't think "Cinder Spires" was ever the sci-fi name; i think it was always the steam punk project. And I'm sure, since what I've heard about CS is that its a trilogy, that in a few more years, he'll be free to work on the sci-fi project.