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.
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/...
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.
[1] https://www.reddit.com/r/apple/comments/1xuzif/what_ever_hap...