To be able to fully compete with the Safari browser, Google called on iOS-developers to integrate their applications with Chrome. But, still, remains the main Safari, even all the links in the applications are opened by default in the browser from Apple. Developers can act in two ways: add an application built-in browser on Apple's WebKit engine and redirect users to Safari.
In the second case the majority of users do not return to the application from the browser, but the reset button in Chrome would remind them of the application. Since the position of Chrome on iOS is very weak, then the constraints of the operating system does not allow you to install third-party application as the default browser, so users often have to open links in Safari.