why is it that

[Sarah]

There are a lot of pirates that made guilds just for themselves, and are not really active guilds. As of right now, active or not, POTCO keeps those guilds in place, taking up those names for said guilds. So, the list of available names gets shorter by the day.

Your best bet is to check out the disney dxd channel, and search the guild section for names that are already in use, and hope that the next one that you want is available.