Their goal with Hangouts was a unified communication platform that tied Hangouts into G+, Wave (when they originally announced their plans, this was still a thing,) SMS, alladat. Basically a single hub that you can use all of Google's communication tools in.
I spoke about it a little bit earlier in this thread, but Google is in a catch 22 scenario. They want to do things to unify the Android experience, but they can't force the issue because other companies might see it as an anti-competitive gesture. They know and understand the value of a closed source distribution model like Apple has, but they can't make closed source moves like pushing a defacto Android messenger for all android devices because that would open them up to litigation from software companies and OEMs.
This is one of the issues with open source development for large corporations like Google. They HAVE to ensure they don't step on any toes, otherwise they get sued.
