Just one data sample : every client I've worked for (we do contract development and consulting for [mostly] startups) over the past few years has deployed on Ubuntu. I can't remember encountering a debian box since the late 90's. As to why? I don't know. Mind share probably, and a load of "to make $X work, go get an Ubuntu machine...".
For our own in-house deployments we have been migrating from CentOS to Ubuntu, again "because everyone uses it" and because it seemed to offer the best chance of working on new hardware at a time when hardware was changing rapidly a few years ago (the introduction of USB3; Atom SOCs, etc).