Why no multi-effects or midi synth add-ons for rear empty space on KPA?

Supply and demand. Very few people are interested in this, and those who are, can probably find a way to fit a synth pedal into a rack of something similar with a kemper. Also, im really not sure why you would want to use an external multifx pedal with kemper, since it already has more effects built in than most pedals anyway.

I have no back space due to owning the powered head. It wouldn't make sense to design a unit that would only work with 1/3 of the Kempers out there. (gotta remember the rack units, too.)