Generally speaking, Ubuntu is often easier to work with than other OS's and as a result, many guides and how-to's are geared toward it (or Debian) and it's package manager offerings (
There's not a specific reason why Ubuntu is used, though it's a pretty solid and standard choice for an OS when you don't have specific requirements that may be better served by another OS.
At the end of the day, it really comes down to preference. I've used Ubuntu for a very long time, so I'm a bit more partial to it myself (or Debian, which is what Ubuntu is derived from). If you've used CentOS for the last 10 years, then you may prefer it -- the same for FreeBSD and others :-).
That being said, the one-click images are really meant to get you up and running quickly. You can, of course, setup CentOS or any other OS and do the configuration manually. That's always an option and is generally preferred if you need something a little more custom.