A lot of people will leave because they see it as the group has very little chance to win so its faster for them to take the 15 min cooldown and queue up with a new group then wipe with a group they think they won't win. There are stuff in DF that pretty much make it impossible to win. I finished 9/9 relic weapons and I seen some pretty horrible stuff on Titan HM, such as dps using all 20-30 gear, and a lv 40 weapon, stuff like this prevents you from mathematically winning the fight.