Originally Posted by
Niwashi
Easy. Long queue times.
Most people do something else while waiting for a queue to pop, and that something may be an inventory intensive activity like crafting or fishing. You get your inventory mostly filled up doing that, and when the queue finally pops, the first thing that comes into your head is "ooh quick, finish and switch to my job for the dungeon so I can accept the queue". Not everyone thinks about checking whether they also need to go find a summoning bell to drop off the stuff they've just been gathering before they click on that "Accept".
Then if you get into a dungeon with a few open slots left, and think you're ok, those can get filled with random junk mats dropped from mobs along the way before reaching the boss and that treasure chest you'd been hoping for. (And you're likely too busy with the running fight to notice those random drops or how they're filling up your last remaining spaces.)
Admittedly, it probably happened more often when we only had 100 slots (most of them perpetually full with armoury chest overflow, baits, currency, and consumables, so only a few slots left to work with for crafting/gathering or looting). The upgrade to 140 has helped quite a bit. But it's still the same issue, and still something the game should address (just as every game should and many do).