The only reasonable choice.
If they chose Google or Microsoft, they would have two vendors to deal with, one the OEM (Samsung, LG, Nokia, whoever), and one for the OS (Google or Microsoft). In case of Google, they'd have a vendor whose business model is basically selling advertising, while Microsoft is a niche player in mobile at the moment with a big question mark over the future of its products.
Seeing how the cost of the actual devices is only a fraction of the entire contract, the fact that Apple's devices are sold at a premium, isn't a big issue for them.