I was part of the Ubuntu decision at GitHub, and it was based on the fact that Ubuntu gave you much longer support after the next release. Debian (at the time) only guaranteed even security updates on previous releases for one year after a new release.

When I made the decision for my shop, we chose Ubuntu because of the guaranteed support cycle. Debian's support windows were too short, and we need to be able to plan years ahead of time for when we must spend the time to upgrade the OS. It takes a lot of testing and tweaking, and Debian didn't give us any certainty beyond a few months out. Ubuntu gives us a five year guarantee (and they've been sticking to that for six years now). That allows us to be a lot more confident about making plans for the next year or two.

Because Debian doesn't provide enterprise support. Let say that you've got a critical bug somewhere, do you really think you can tell your boss 'Yeah I asked on IRC / forums / mailing list to get an ETA'

Source?