particular dungeon if this is the case

If you choose specific dungeons instead of the Random Dungeon option, you should never get an in-progress dungeon. We understand you may be looking for a specific dungeon boss because of a loot drop, achievement, etc., so it may be in your best interest to queue for that particular dungeon if this is the case.

We don't want to make it gold wow too easy for players to skip over in-progress dungeons. What would happen is a Dungeon Finder group would virtually always be abandoned as soon as anyone leaves the group.

Also, I think right now you're seeing more in-progress dungeons pop because, in general, the dungeons are more challenging now than they were when the Looking For Dungeon feature was first introduced in Wrath of the Lich King. Some players may get frustrated and bail at the first sign of trouble, instead of realizing that everyone may have to work a little harder than they're used to in order to conquer a dungeon.