People would find the most efficient dungeons to spam and only do those anyway, and the queue for anything else would be ridiculous. Also, if there was Lv50 of all dungeons, there's the possibility it might spread out the community and the DF queue too widely, as people can only queue for up to 5 dungeons. And then there's the issue of when inevitably higher token tiers are added. Then add "extra hard" mode for that tier?