Why aren't YouTube videos playing on iPhone anymore?

So up until a few days ago I've been able to play any YouTube video on my iPhone 4s as long as I was in "desktop version" for YouTube. I use YouTube's desktop version on my phone because a lot of times the mobile version does not support the videos I want to play. But recently, for most videos in desktop version, I keep getting the message, "sorry this video is not available on this device". Why did this suddenly start happening and what might I do to fix it?

Your view history has raised a red flag with the NSA and they have shut you down.

YouTube has TONS of bugs for iPhones, sometimes it says I'm not authorized to watch a video, or it's in a different format. YouTube is cray cray!

Whoever you're watching on YouTube just happened to change his/her targeting audience in order to bring in more revenue. Many more Youtubers are doing it now. I'll explain. Moderately large to large YouTubers tend to display adverts on their videos because they receive source of income this way. Though, they do not receive any money whatsoever if the advert was seen on a mobile device. So what the YouTuber will do is change the audience settings in hopes that those watching on phones and other devices will find a nearby computer, leading to once again, more revenue. I believe they changed come policies regarding adverts and revenue, more heavily influencing these actions.