Currently, Shuttle only sells its motherboards as a part of an XPC barebone, comprising of motherboard, chassis, power supply, CPU cooler, and accessories. Customers who already own an XPC system but who would like to upgrade their motherboards to support the latest features, find that they need purchase a whole new barebone without the option of reusing parts to reduce costs.
Particularly for enthusiasts and hobbyists, being able to pick and choose your parts is half the fun and a big part in deciding what to pick. I'd be much more inclined to pick up an XPC motherboard if I could get just the board. Interesting stuff.