They expected to add a TON of complexity to the messenger product (they announced messenger as a platform at F8) and they needed a way to decouple the already complex main FB app. It buys them a bunch of things, like easier maintenance, extensibility, and ability to push updates independently among other user experience benefits.
They also didn't want to split the user experience of using Facebook messenger on mobile and hence the forced switch.
It was only a big deal because of the surface area of the world that the main Facebook product (w/ messenger) covers. I guarantee you that if Google did something like that no one would care (at least not enough to give 20,000+ 1 star reviews)
