I have 15 characters in total, and I have never been in this raid before.
Reason being: I've never unlocked it. I simply never make time to.
It just takes way too long to assemble the Sigil, and you'd need to repeat the Necro 4 quests several times, or farm the heck out of the Orchard rares before it's completed. In order to gain entrance you literally need to stop everything else and focus on repeating N4 content for a significant amount of time.
Now the problem is; this isn't Gianthold. And LotD is generally not liked by people.
So I'd say the flagging mechanic needs to be rethought in Necro 4 to be more realistic. Either lower the amount of pieces the Sigil is made out of, or drop it altogether and turn the quests themselves into the flagging mechanic.
At the very least make each quest have a 100% chance of dropping their sigil.