Well, Grob was the only established RP-PvP server prior to launch, so many of the retail guilds went there and set up shop and made plans for weeks before the game launched.
Deviate Delight was opened suddenly several hours after launch, and not many people knew about it because they were already sitting in a Grob queue. Those people decided to stick it out on Grob for the most part, since that’s where they had already started playing.
Now, I’ve rp’d a bit on both servers, and it seems about the same; General, Trade, etc are mostly filled w/ OOC chat (this is the same on every RP-PvP server I’ve been on, though), but if you make a bit of effort by either just RPing yourself or going to look for it, you’ll find it.
I was a Ravenholdt native from day 1 of TBC up until about the end of MoP, when my guild decided to move to Emerald Dream, where I’ve been ever since. On any RP-PvP server, you have your main population who is there for PvP or just to play the game, and RPers kinda doing their own thing, but they are definitely there!
On DD, I’ve had short impromptu RP sessions pop up in a lot of places (waiting on the zeppelin, hanging out in the Crossroads inn, slow-walking around Orgrimmar, etc) and it’s very refreshing. Sure, we have a few trolls and refugees who came looking to escape their queues, but a lot of Grob population is the same who rolled there on day 1 when every other server was full, so it’s kinda the same story.
Wherever you decide to go, I hope you have a lot of fun and make friends!