It's a production issue, from what I was told. Also, apparently when the Shimano stuff is produced it's on a first come first serve basis with no back order process, so it's a matter of seeing it right when the vendor lists it and grabbing before someone else does.

I ran into the same thing - I bought a Specialized Allez Sprint Disc frameset I wanted to outfit with ultegra - couldn't do it and couldn't give me a timeframe. Ended up with a Di2 Tarmac instead - a "nicer bike" no doubt, but also quite a bit more than I needed to invest in a second bike.