It's great to see them follow through with this. I remember when Steve Jobs went on stage and said that FaceTime would be an open standard. Haven't seen that happen yet.
My understanding is that the patent nonsense forced Apple to transform FaceTime from a peer-to-peer protocol into one that transmitted all data through centralized servers, specifically Apple's servers.
That meant that if the goal was for interoperability, Apple had to either provide server capacity for a bunch of other people's traffic, or they would have had to come up with a way to federate the system.
Federation certainly could have been done, but that's a lot of sudden extra growth in scope compared to just taking what they had and releasing it.
Re FaceTime, I recently read an unsubstantiated but plausible comment that they'd run into patent issues that caused a redesign that caused issues open sourcing it.
and according to someone I heard on a podcast, many of the engineering leadership heard the open sourcing comment for the first time while Steve was up on stage as well.
Not sure which particular episode (you might be able to find it by looking through old shownotes) but this was mentioned on Accidental Tech Podcast, I think by John Siracusa (of really long MacOS review fame)
FaceTime has been the subject of multiple patent lawsuits, which is what has prevented it from being opened.
See: http://www.bbc.com/news/technology-20236114 and http://www.techzone360.com/topics/techzone/articles/2014/09/...