The major reason for choosing Debian is the stability and "Long Term Support" which is very suitable for servers. Debian Long Term Support (LTS) is a project to extend the lifetime of all Debian stable releases to (at least) 5 years. A server should run "forever". Set and forget. All software in the distribution is not the latest but the most stable.
Another reason is that its low footprint. 512 MB memory and 2 GB hard disk is enough for a server (according to debian.org). Which makes the VPS cheaper and faster.