Google did not renew Apple’s license to the Apple Youtube app, so Youtube is not available on iPhone anymore. (Well it is still available through third party Apps like Vevo)
Some speculation is that the ancient apple app (natively built into the iPhone) carries no advertising so it is in the best interests of Google and Apple to not renew the license as surely a new Google App would generate huge advertising revenue, which would benefit both Apple and Google financially.
Apple already stated that there will be a new Google Youtube App available shortly.
Apple also recently stated that Google Maps would be replaced by Apple’s own Maps.