Because obscurity is the best security!... right? FCC knows what's best for everyone.
Isn't DDOS mitigation an area where obscurity is the standard? I am no expert on this but it seems like most providers keep the info about how they filter traffic pretty close to the chest.
Alternatively you can usually let Akamai or similar advertise your IPs, and let them help with the load, but ultimately that's just distributing traffic to more devices so you can still check IPs against a blacklist.
Since we're talking about the availability aspect of security, there is hopefully no obscurity involved (confidentiality on the other hand is nothing but obscurity). And really, an obscurity technique like using alternate ports won't even help you much because that still hits your firewall and requires processing on each packet.