I think they merge servers based on their assessment on the active level of the servers. Many old servers still have a lot of players active but they just all join into 1 or 2 groups only, which lessens the competition in that server. And those servers want a merge to get more players and more competitions. Because of that Oasis decides to prioritize merging those servers that are literally "dead", like there is literally 5 or 6 active players on those server, instead of focusing on the old servers that want more competition