I think the DESFire being 3x13 makes it difficult to pair with the much smaller 2x12 chips in a single implant. The xNT and the xEM are the same size, making it easier to fit into an implant without rattling around.
Then there’s cost. DESFire is expensive, and pairing a DESFire with an xEM would be quite the expensive implant. And is there enough demand to even invest in such an implant? I don’t imagine the DESFire are exceedingly popular.
I get it though. I’d much rather have had an xM1 instead of the xNT. It’s be much more useful being able to wipe the UID off of the HF portion of the chip. But again, size mismatch.