I suspect its because containing Google hinders the web experience for people more than containing Facebook would. People have mentioned in threads here that Capchas would become frequent, Google login (which probably is more common than Facebook login) wouldn't work. The addon would make the browser of an average Joe worse than Facebook containers. And when Mozilla is grasping at straws, trying to win back market share, it'll have to really debate whether or not they would want to go forward with this.
Doesn't help that FB is getting a bad rep these days and its more mainstream to hate on it.
Then an interesting question is IF the Facebook container stemmed from principles 4 & 5 from the Mozilla Manifesto applied to the dominant social utility; is there a reason other than money that the same principles would not lead to a decision to build an add-on that gives people an option to contain Google? It's certainly not a technical limitation.
It seems natural to posit that money is the issue but is it so easy to believe that a foundation most of us trust would compromise its core values?
As mentioned elsewhere hacky add-ons cannot be the end solution, and will not be widely adopted unless incorporated as defaults.
Doesn't help that FB is getting a bad rep these days and its more mainstream to hate on it.