Because consumers exercise choice and use multitude of products from different providers. Sometimes we also pay for those services. It is discrimination against consumers if they are being forced to choose platforms based on eco-system support and not their choice. There is no denying that Windows Phone is a promising platform, but app providers like Google keeping their apps walled to Android or iOS is not helping the consumers. The rivalry between the vendors is their own and consumers shouldn't be dragged into it.
Google has made public statements about not supporting Windows Phone platform, even to an extent of pulling support for the Exchange Activesync protocol. At the end, the consumer suffer.
Google will do great service to their users and consumers by offering some products on rival platform, notable Google Apps, Drive, Google+ and Google Talk.