Why don't exist any Proteus 2XR EMU Kontakt Plugin?
E-mu Systems is currently owned by Creative Labs. They own the intellectual property, and would need to have a reason to create a Kontakt compatible version.
It does exist, just not by Native Instruments.
and more (not the 2xr though)
Digital Sound Factory propose it, but only the Proteus 2, not the Proteus 2XR.