I'd really have thought it went without saying, but Blackrock is cross-server'd with Onyxia for the purpose of increasing the player pool available for PvP and reducing queue times by having them counterbalance faction imbalances in the queued people. That's why the server has always on Mercenary Mode. Onyxia gets shorter Battleground queues, Blackrock gets more people to play against. Aside how easier it would be to abuse arena quests, it should be obvious why more quests involve Battlegrounds. There would be close to no point in the cross-server if the focus was on arenas.
Regarding this fable that Onyxia requires less effort, let's even ignore the absurdity that is assuming every single player would be able to have a group to complete raids with. How would you feel about randomly receiving one of the following messages - among the many others that would fit - when you went to get a new piece of gear?
"Your group successfully cleared the raid, congratulations! Unfortunately, someone else beat you in the GDKP. Better luck next week!"
"The raid is over, well done! But you're a new member of the guild, so an older member got the item you wanted. Stick around a couple months and that might be you, though!"
"Yes, there's the winning roll for you... wait, what just happened? Oh no, the raid leader ninja'd your item! Ah well, maybe next week it doesn't happen, chin up!"