Thr reason it is pro Debian is that is what the FreeSwitch developers recommend. Because of that, the FusionPBX developer also uses Debian. This means Debian is the tested version. All the Centos stuff is usually community contributed and those community members come and go so often the Centos stuff is untested and brings loads of problems over time.
I use Debian. Why? Because that is what is recommended. If Centos where recommended I would switch to that. I came to FusionPBX to get a working production PBX so I use the OS that will give me and, more importantly, my customers, the least pain.