As a tank my queues were so fast that I didn't notice there was an option to join dungeons in progress until I reached level 50. DPS know about those options because they've had lots of time to stare at the queue UI.

Also I agree that because you get fewer rewards for doing an in progress dungeons (some bosses [and their loot] may already be gone, less xp from trash if you're leveling), tanks/healers prefer full runs, while dps take whatever they can get.

Maybe the checkbox for in progress should be removed (everyone joins in progress) and if you join in progress, you get an extra drop just for you if you complete the dungeon.