What makes writing presumptuous.
It wasn't originally. So yeah...
Originally there were only two DC's (NA), and Balmung was on Aether.
There was no RP datacenter. There was just Balmung as the RP server. And its all unofficial anyway. And even then, the servers were adjusted when they made the split so... Crystal was made of servers from both Primal and Aether.
Mateus became the secondary RP server before even that though. Hell, years before because of how long Balmung remained locked to new players. Part of the reason Crystal was given so many of the smaller worlds was because both Balmung and Mateus were massive on their own and doing so would even out the population somewhat. That said looking at their forum join date they definitely predate that.
Doesn't really matter if you think it's ridiculous or not. Bulk of RPers who either moved from or couldn't get into Balmung moved to Mateus. That's the thing that happened. There are some small groups on other servers and other DCs that also RP but that is just how the trend shook out.