The issue with Windows Phone was that it was a Microsoft venture, and Google was already paying MS around $5 in license fees for every activated Android device [0]. They weren't in any hurry to make WP more competitive by offering a Youtube app. I had a WP device in 2011, and there were third party apps (MetroTube I think) that were like Newpipe in that you could watching and download YT videos for offline viewing. Not surprisingly, Google changed their APIs and broke the download capability.

That said, Google didn't build third-party YT apps for BB10 or WebOS either.

[0] https://www.computerworld.com/article/2475440/microsoft-gets...

Why and how?

You can look at the difference between iOS and WP. Since iOS was a big market Google wanted [and wants] to be in they made the YT app. And since WP was small and late to the game Google had the upper hand, because MS wanted any users more than G wanted the small number of WP users.

https://www.theverge.com/2013/5/7/4308662/youtube-windows-ph...

Therefore if there were a credible new platform that Google wants to be on, they will develop the necessary apps.