One reason is because the quest is very challenging. Even level 25s might have a hard time holding an invulnerable elite marilith (that randomly loses agro) while the rest of the party deal with the efreets.
The second reason is that, even with the quest being free, the quest giver is hidden in a corner of Inspired Quarters.If you don't look the map for the golden chalice, many people miss her completelly.
And honestly, I see that as a problem. I'm a firm believer that an elite completion should be earned, and not granted. Just raising numbers turn the elite completion into a gear check, and not a skill check. It should be a mix of both.