I can speak for myself - I use Ubuntu on my laptop and I'm doing most of my software development on it.

As such, it makes sense to use the same environment in production. You have less surprises that way and you also learn to configure the server just from day to day maintenance of your own laptop. And some things are the same in Debian and Ubuntu, but some things are different.

Regardless, Debian is awesome, and its heritage is one reason why I love Ubuntu.