There may be other reasons to make it more popular for servers. One of the major thing is it is based on RHEL and the user base. RHEL is quite a big player, the OS and management tool is well known and with their large user base and volunteer support it is easy to find solutions for most of the issues. Additionally the standard training for Linux people take is the certifications from RedHat and many people are more familair with RHEL tool sets when it comes to server management toolsets.
The main difference between CentOS and Redhat is you get support with a Redhat subscription. CentOS releases the same patches that Redhat does, so if you do not need support there is no need to use Redhat. But do not get this wrong, think of this as the same type of support Microsoft will give you for a copy of Windows.
Many corporations use Redhat simply to say they have an certified enterprise linux.