There may be a good reason for that.
Depending of the version of the paravirtualization-capable hypervisor used by your host provider, you may need to use/install/update an older OS version that is compatible with your hypervisor.
One of my favorite hypervisors is Xen.
One of my favorite OS versions is CentOS.
So, If you need an older version of CentOS for your VPS, then check out http://mirror.teklinks.com/centos/
It took me a while to find that resource. Most CentOS mirrors seem to only provide the lastest version and README that says the version you're looking for has been deprecated.
I hope this saves you some time.