Remember that inside the enterprise version, you have full control over all the organisational structure - essentially giving you an extra level of granularity on permissions. That's quite important for many companies.
For these companies, using public GitHub isn't even a question. It would cost far more in terms of lawyers, managers, etc to approve use of public GitHub than to just buy an enterprise installation for a department.
Also, just because they have (and sell) a product that does this, doesn't mean that they use it, that it's appropriate, that their devs want to use it, etc. Separate departments in companies of these sizes act like individual companies, IBM even has the term Blue Dollars for money that they spend on other IBM services.
Obviously winning someone like IBM and HP (I don't know what they use) would be big.
But I see most companies using GitHub private organizations (paid per repo) even when they are sometimes quite large. I might be totally wrong here, but I'm kinda interested in what the split is.
Obviously every software company having a huge chance of keeping their private source control code in you is a big deal, and even "monetize way later" makes sense here considering the dominance they are showing.
And mostly showing without any major improvements to the product (.com) in a LOOOOONG time, but I will say the stability has gotten much better, and I do appreciate that at least.