Google will make an official YouTube app for Windows Phone. Not only that, but they'll also make one for Gmail, Maps, Google+, Hangouts, Docs and Drive. I don't have any insider information or anything, but it boils down to Google's incentives.
Yes, Google wants Android to take over everything, i.e. phones, tablets, PCs, televisions, cars, etc. But pushing Android in of itself is not the aim, the aim is to broaden the data collection web as far and wide as possible. Whatever Windows Phone's market share, there will be a time when over *100 million* people will use WP devices. If the 15+ new OEMs succeed in terms of sales, that time could be quite soon.
There's no way a company such as Google would not have tabs on some 100 million people all over the world. In fact, that would create the existence of a dark zone that no other company akin to Google would ignore. While advertising is their core objective, there is another market brewing for collecting and interpreting consumer data.
Note, I'm not saying Google would offer first class experiences on Windows Phone, but it will ultimately offer enough to keep people signed in and using its services. You just don't ignore 100 000 000 individuals.