Too many existing players leveling Rogue/Ninja and not enough new players or old players without a healer and tank leveled leveling healers and tanks. It's also another bad thing about relying too much on instances, if dungeons were open world you could just walk in solo or in a party and wouldn't have to wait for a queue, without a queue popping there's not much else to do except FATEs, leves and your 10 or so log entries. Sure people would still crowd in open dungeons, but that already happens in FATE zones and it wouldn't be that different if they handled mob respawns and server loads correctly.