I did try going through to see it it was mentioned why people quit at the beginning but...
Obviously those that don't want to spend time in that dungeon quit...or have to leave.
In quite a few cases, people are funneled into a dungeon that no one in the party needs. No worries if someone wants to quit. As a person who has started out as a DPS I know the feeling of long queues and will stay if people need it. But if no one needs it, the game has no way of determining that I quit for X reason - so punishing someone for a longer time than already there doesn't discriminate why a person left a dungeon.
Speeding up queues for friends before a player has to log out. In the case of a lot of dungeons, queue times are ridiculously long, especially MSQ especially. It's faster to queue in for a DPS if a healer is in the party for that content and go on without a healer (they eventually get replaced a good number of times) and the MSQ is quite easy with them missing anyways.
Another reason are bots. Sorry not gonna waste any time being in queue with bots.
Now if they can base a penalty system on people quitting on those that are IN NEED of said dungeons and 2 those in need players are not bots. I'm all for it. But if people are quitting because the instance a bunch of 50 players are getting is Tam Tara (and they don't need it). I don't see taking the regular penalty is a problem.
This isn't going to solve everything because Yoshi P has designed the current state for players to be able to take longer breaks from it anyways because he does want players to go play other games and come back to it, instead of being on it so much you burnout and quit. So a longer penalty isn't going to work. Forcing a dungeon on al lower level dungeon until you complete it, is going to make me avoid that roulette for a long time, and getting tomes isn't that hard. Leveling roulette is useless unless I was leveling a job :P