First, they would have to add a interface into the hardware. Second, they would have to decide if they are going to put in a cheap interface or a pro-type interface which would drastically up the price. Since the Kemper is pro-type then the latter could add another $500+ to the cost of the stage. I believe the Stage was designed for mostly stage use and not necessarily a studio piece. The toaster and rackmount would fit better with a built-in interface (they probably have the room for it). But there is so many options for an interface that Kemper would only satisfy a limited few - so, in the end, not worth it.
But when I first bought my rackmount, I wish it had one.
Hm I don't think we can completely rule out that the needed hardware is already present in the current incarnation.