I've used git on repos far less than 2 billion lines of code and it was kind of a nightmare to clone and do basic local checkouts and normal everyday stuff. Out of the box, git's design is not well suited to large repos.

I know there are various workarounds for dealing with large repos but having had some experience using git on big-ish projects I can certainly understand some possible reasons why it wasn't their first choice given the size of their codebase.