It's not that simple. EY's filesystem stuff is designed for read-almost-always, as you would expect for most Rails sites. GitHub does a lot of writing. Growing GitHub's setup would have been a huge undertaking for EY (from what I've read of their posts) and you just can't do that stuff for free, or you run out of money.

(This is just what I've gleaned from comments. They parted ways amicably. Don't stir up drama.)

Yes, but you can't say if they stayed at EY it would be free.

It would also have been free to run Github off of an old server out of their garage. Just not very functional.