Shhh! 
I think this is it. My understanding might be flawed but I think all the heavy lifting and most of the compliance is done by GPS for the *pays. This means that Monzo would have had a much bigger job in getting going with Apple.
(And, incidentally, would probably make it horrendous for the likes of Starling to not only build their own processor but then to integrate it with Apple, Google, FitBit, Garmin… Oh and Badger Pay)