Premise:
The game is getting new quests, but the player population is not growing proportionally. This results in trouble finding groups for particular quests or level ranges as the playerbase is stretched out thinner over an ever growing selection of quests.
Solution:
Recruiting new players out of thin air and removing or disabling old quest packs are out of the question. A better solution is to offer a carrot to focus the players on a narrower range of quests. This can be done by offering an XP bonus to specific quests, for example by adventure pack, level range or favor. We don't want to distract players when a new pack is out, so the bonus would be offered towards the end of each update cycle.
Example:
In the weeks leading up to the release of u17, there would be one week of 20% XP bonus on Silver Flame quests, another bonus week for House Kundarak quests, and a third bonus week for Restless Isles quests. Bonuses could be mixed together ofcourse, or tied in with calendar events (Silver Flame bonus during Halloween) or game events (bonus to u16 quests just before releasing u17).