There are lot of changes in CentOS 7 compared to CentOS6 and older versions. Systemd replaced init, kernel version updated,XFS replaced ext4,firewalld with iptables,Run levels to targets etc etc.
The problem is, it is not a quite good idea to stick on with older versions of Operating System distros for a long time. When ever latest versions comes in, the developer's will gradually stops working on the older versions and starts work on the latter versions. Then put the older versions to EOF 'End Of Life' after a specific terms of period, so the security updates , other kernel patching and major OS updates will be done mostly for newer version. Also the patches for current vulnerabilities with kernel as well as for other OS libraries will not be available for the older versions after a period of time.
Eventually major third parties softwares (like control panels etc.. )that we are using also will not support older OS versions due to security reasons, when the OS is at 'EOF ' status.
Finally it is forced to migrate the data to higher versions.
For these reasons, almost all companies always think to stay updated with newer OS versions.