The drop rate for epic level items is too low. In wow you can get a select amount of epic level items while others are reservered for much harder quests or events. Players do not want to be limited to common or uncommon items for too long or they get discouraged and leave. One way is to limit the amount per week instead of requiring a massive amount of resources. That way not everything reduces down to resource grinding. You already do it for daily quests consider the same for epci item types