The main reason, IMHO, is the fact that new amps only come to modelers when the programmers change the firmware. When the modeler is no longer updated, you don't get any new amps.....ever. The Kemper is basically future-proof, and you get access to new amps as soon as someone profiles that amp.
Nicely done. You managed to ride the line between all the brands without cheerleading for any single one too much. (That you own a Kemper and not the others says enough....but you have an HX Stomp. 
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All these units are about what fits for a given situation. If you're a tweaker and want control over every aspect, then a Fractal is top of the list to try. In the wrong hands, a Fractal can be time consuming and terribly frustrating. If you fall closer to the Keith Richards' approach (the right guitar through a cord into the right amp), then the Kemper makes way more sense.
Like anything else, all of these units can be made to sound fantastic....and like absolute crap. The tools are only as good as the craftsman wielding them AND if that craftsman takes the time to learn its idiosyncrasies. I've seen/heard a Matchless Chieftain player get absolutely schooled in generating good tone by a Fender Mustang III player. Whether they realized it or not, one knew what he was doing and the other did not.
