Because of the network effects of software development. The number of people that use those devices you list is basically a rounding error, so they're ignored by most software development.
Apple has a monopoly on their hardware, and they will likely sell a significant number of devices. This will lead to a lot more development for ARM that never would have happened otherwise.
That, in turn, may make tilt the balance in favor of ARM for a lot of other use cases outside of OSX when other tools, applications, and hardware vendors better support ARM.
