Well, because i like it!
It's updated, you don't have a kernel from 4 years ago in a 2011 8-core box, stuff just works if you know how to make it works, just like on centos.
For example, on the same box, the centos mdadm version takes 2 weeks to build a raid6 system. The ubuntu server version takes 6 hours.
I've also considered to start a new project (10-20 boxes! Holy ****!) with ubuntu server. I've yet to have any problems with ubuntu.
I used to run Centos but the php versions fell behind. This is to be expected given the RHEL philosophy and i'm not into manually compiling updates. Instead of using one of the myriad debian clones(aka ubuntu and others) i went for the original..Debian.
I'm using CentOS atm, but thinking about switching to Ubuntu maybe. I've read it doesn't really matter much, but our host has most other servers under Ubuntu so it would be more practical.
My host offers CentOs. I like it, as the last time I meddled with Linux before getting the box at the host was well over 10 years ago. CentOs offers stability, which I prefer any day over running the latest of the latest.