It appears to be something of a packaging problem. Just take a look at the SR assembly that Pentax has been using in their marketing.
and then with the associated surrounding installation / support package
Then picture this slid into a K body. The sensor will be twice as large, and will need to maintain the same location with respect to the mount.
So, to me - it appears that it comes primarily down to packaging - and finding the amount of room for the assembly to need to move around within. Also, the supporting structure is going to need to be a bit larger to accommodate the larger sensor. I get the feeling that Pentax is trying to maintain a similar body size - which is making things a bit difficult to accommodate the increased sensor size along with the necessary room in which to allow it to move to support SR.