I've asked about this before on UKC, when there was a BBC video of them coating the tissue. No one seemed to have the answer, but I guess there is some issue with the material being about to flex, making one side of it expand and the other contract. Maybe the nano-coating cant cope with it.
I guess it's because it won't be at all breathable, making a fully waterproof jacket is easy, just wear a bin bag, being able to let your sweat out is just as, if not more, important.