The source code was never very private in the first place—it's easy to de-obfuscate it. More importantly, GitHub doesn't want to support and deal with open-sourcing their core. There's some amount of irony here considering that GitHub is the primary place for other open-source projects to be developed, but it's important to remember that GitHub makes their money from private team repos anyway.
Making the source code open...
1. is not equal to making it open to collaboration.
2. does not mean they can’t keep making money from it.
What incentive do they have to keep it open, though? People who already needed to read the source code (like security researchers) don't need the complete unobfuscated source in the first place. It would likely cost GitHub money in frivolous support queries.
Security research might not require unobfuscated source code, but it does make it easier.