The issue is with how the FF14 den matchmaking system works.
Early on, there was no matching by ranks and early players would get matched against more seasoned veterans and would get absolutely destroyed. In addition to being more experienced, higher rank players had more skills unlocked and morale on their gear, further widening the gap. In an attempt to balance this out square enix revamped the match making system so that groups would now be balanced. In order to get an instant-pop both teams are required to be within 11 total pvp ranks of each other.
When this change was first implemented, den was still popular enough that it worked early on. Newer players had better experiences as the teams were generally more balanced.
This change had an unintended side effect though. As fewer people queued for den, queue times dramatically increased. The way the system works is if your group can't get matched, every 3 mins or so it expands the group matching algorithm to include a wider pvp rank tolerance. With so few people queuing, you now either never get matched, or when you do you have waited so long it's likely you're going to get matched against someone much higher/lower rank anyway, the only difference being you had to wait 30+ mins.
While experienced players still have a huge advantage, a lot of the original problems no longer exist. i80 den allows people to queue for a mode where gear doesn't have nearly as big an impact, and changes to morale made PVE gear actually viable for PvP even in uncapped. At rank 1, you now have access to all your pvp skills and can freely re-allocate them vs. before where you had to unlock each skill 1 at a time, and god forbid you messed up it was extremely costly to reset your skills.
Anyway, this is a long-winded way of saying that even if you got 7 other people to agree to queue up today, it's unlikely you're going to get fast pops. There will inevitably be significant rank differences and you'll be lucky to get any matches at all.
The only reason the solo queue event works is because everyone is required to be rank 43+ to do it. If everyone is max, or very close to max rank, the rank matching is a non-issue and we get instant pops. Even then though, we often have some joker who doesn't know better who queues up rank 22 and it's jammed up the entire solo queue to the point where everyone has to wait 15 mins to get a pop.
Your best bet to get the challenge log is to get a group of 8 people on gilgamesh, divide into groups of 4 with balanced ranks (total difference in ranks of 11 between each group), and queue up 10 times on some different language like German. You'll get instant queues, and hopefully you'll have some good matches as well.
If you can't get a group of 8 on your server, you can also try solo queuing for level 30 den English only. From what I've heard, this is the den bracket that pops the most often and you should have the best luck there.