Defend quests have one thing in common: The player base avoids them at all costs.
Why?
1) Failure possible. Noone likes sitting in threnal east 3 and failing in the last minute of a 15 minute quest and getting nothing for their time. Even worse, quests where failure is possible are usually LESS rewarding than similar level quests, but that is covered in #3.
Fix: Spread the exp out per wave. Devil assault was pretty well done in this manner. Make the end quest exp no more than 1/3rd the total quest exp. If someone can get 2/3rds of the expected xp total with a failure, then they would likely try again. If they get nothing, they would likely say "screw this" and never run such a quest again. Additionally, such a system would be more forgiving to harder and more interesting defender quests rather than the recent ones such as finding the path that only pretend that the objective can be destroyed by the enemies (It can't as far as I know).
2) They are BORING. Most of them have mob spawns that take forever. Finding the path and weapons shipment are the worst offenders. The mobs come 3-4 at a time with about 30 seconds between spawns, which means the players wait 29 seconds between waves. Now sure on elite they are moderately more interesting, but a decent group still spends a big chunk of time waiting around, and quests should be fun on both normal and elite.
Fix: Waves spawn right after the last monster of the previous wave dies. If players need a breather, they can kite the last mob for a bit. Keep a timer so the players cant kite forever though. Baudry pt 1 can be pretty exciting the first time as the spawns are many and they come pretty fast. On elite with level appropriate characters I sometimes have trouble soloing it, which is about how things should be. The longer the timer, the less the quest feels like defending an objective, and the more it feels like inviting your monster friends over for tea.
3) They take forever and give almost no exp/loot. Weapons shipment would never get run without the bauble. Threnal East3 and Kobold assault are literally the quests with the worst xp/min ratios in the game (not to mention being in the bottom 10 for loot/min as well), and they lag behind the good ones by a factor of 20-40.
Fix: Make defender quest worthwhile by bumping up the exp. Anything less than 1500xp/min means defence quests will not get run unless they have stupidly overpowered loot (bauble in WS), or are required for flagging (finding the path), or its an F2P quest (kobold assault). Usually I would say 1k/min is good enough, but since defender quests have the possibility of failure they deserve better. The higher the likelyhood of failure, the higher the exp reward should be. Additionally, there should be 1 chest/5mins in quest. Any less, and they should be 1 or 2 loot levels higher than the quest.