Two entirely different mounting systems.
The hardshell Hobies support the mast and sail entirely by means of the mast sleeve. This requires no stays or additional mast supports.
The inflatables support the mast completely with the 3 stays. These triangulate the mast, just as a tripod supports a camera. Needing no additional support from the mast mount, a sleeve is not necessary. The little pocket only needs to keep the mast base from dancing around the cockpit and provide a puncture proof seat for the mast.
Each method has its advantages. The hardshell mast is quicker to set up -- just drop it in. On the other hand, the inflatable mast is adjustable for rake to help offset weather helm. Both styles dump wind when necessary to augment boat stability (by different methods -- one by bending, one by leaning).
They both work well, yet each is best suited to the different style of boat construction that accommodates their use.