[Hengjie]

For a while, before iOS 8, Safari would be the only application to take advantage of the Nitro Javascript Engine. No other web view (Chrome for iOS, Facebook, etc) was able to take advantage of the increased JS performance. I had heard that it was due to the way Nitro accessed memory within iOS that prevented it from being safely used by other developers.

[rdsnsca]

Apple does this when testing a new feature in both iOS and Mac OS. They make it a private frame work and test it in 1 or 2 of their own apps until the feel it is fit to release for everyone.