the Eigenharp are not usb midi class compliant (which is required for iPad connectivity , e.g. sensel, MPE then sits on top of this midi layer) , they run have their own usb protocol - which allows for higher resolution, lower latency, less jitter....
BUT means eigenharps need an application/driver (like EigenD) which then talks this protocol, and then either hosts a sound generator, or converts to MPE/OSC or whatever is required.
unfortunately, Apple are very protective about drivers, as they can harm reliablity, performance and battery life - so their process make it difficult for manufactures to do this, as John already pointed out.
there is some good news though...
as computing devices are getting smaller and more powerful, there are much smaller devices available that are capable of interfacing with Eigenharps..
Recently Ive been using Eigenharps with Raspberry PIs and also an Organelle. as demonstrated on this video - as you can see , these not only do the 'driver' but are also capable of the sound application too - I actually prefer these devices over an iPad, as theres some really interesting patching environments (Pure Data and Supercollider to name just two)
given the rate of technology progress, im sure it wont be too long before, an Eigenharp to MPE 'dongle' could be created.