Do you suppose it is because they feel the enthusiasts aren't interested in mATX AMD boards? That doesn't seem to be the case with Intel Z77 or X79 boards. Plenty of mATX boards for that chipset out there.
Been keeping an eye for such a thing myself for some time. Gave up. Came to conclusion AMD decided small form factor only for APUs. Tech trends in the last 50 or so years is to squeeze higher performance into smaller packages. Locking their highest performing chips strictly onto physically larger packages (when SFF is technically feasible) I consider a major blunder... as if they're purposely telling one of the more profitable sectors (non-OEM hpc builders) to take a hike. If OE builds favored AMD I might be able to grasp such an argument, but OE's overwhelmingly favor Intel (with no signs of that changing). Makes no sense whatsoever to me. AMD isn't doing well. Vulture capital circling. Someday I'm sure this will be included as one of a multitude of little nails, with the single biggest stake to the heart being AMDs ridiculous pre-release hyping of BD into something it wasn't.
Maybe, as knightdog says, it is the MB manufacturers that have made that decision. Not AMD. Whatever the reason, it forces everyone into an Intel system for those building with small footprint MicroATX Mid Tower cases. And a little less revenue for AMD.