It's cost, size, and regulatory. Existing current probes that are magnetically coupled and thus don't require an electrical connection to the mains (which would be a safety/regulatory nightmare) are big: http://www.tek.com/current-probe
The bigger probes that are meant to go on bare, live bus bars and such are probably mostly air to give the operator the required safe handling distance... probably not an issue for a clip-on probe that goes on insulated wires?
[0] Technically a P6021 sold to Philips who bundled it up with a matching amplifier of their own design.
I think we are saying similar things though. I worked on this project with Verdigris (though I was not personally responsible for the design of these probes) and, to your original point, the challenge is fitting these at a reasonable cost while achieving the desired accuracy. I guess I'm so used to low manufacturing costs being a given that I tend to focus more on the other aspects of the problem.