Borror0 of the DDO Wiki stopped by my post about picking up the unpopular Catacombs adventure pack to offer some hints on why the quest is unpopular. Now I'm going back through that quest, and the more popular group Shan-To-Kor (STK) adventure pack, on hard mode, and the contrast is a bit interesting. Fundamentally, Catacombs plays like a solo quest chain, while STK plays more for groups.
Catacombs: 8 quest instances, but most are short, I'd guesstimate 10-20 minutes each
STK: 3 quest instances, but all are longer, and took me 30+ minutes (also one optional side quest located relatively near the main storyline, and included in the adventure pack purchase)
Overall, both quest lines took me about the same amount of time actually in dungeons fighting stuff (about 2 to 2.5 hours on my first playthrough).
However, the Catacombs is broken up by the frequent trips outside of the dungeon to talk to NPC's and advance the quest story. For a group, this is probably irritating, as it means that everyone must wait for the entire party to go talk to the NPC and then move on to the next instance (assuming that no one decides to take the in-between-quest opportunity to bail out).
By contrast, STK is more of an oldschool dungeon run romp, with no need to talk to any NPC's once you're started - you can run the three dungeons back to back to back if you're so inclined (though this is likely to result in some very full bags if you're doing it solo and actually looting stuff).
Progression and Pacing
Catacombs: The first six quests are normal level 3 quests. The seventh is an "extreme challenge" level 3 quest, and the final boss fight cons level 4.
STK: The three quests in the main questline are levels 3, 4, and 5 respectively.
The DDO level curve, true to the pen and paper game, moves pretty slowly, but each level has a comparatively larger effect on player power levels. No one will gain two whole levels in the course of a single run through STK. A group of mostly level 4's that runs all three quests will have an easy round to warm up and get used to each others' quirks, a normal round, and a slightly tougher final round. A solo player will almost certainly be attempting one or more of the three quests at an inappropriate level, unless they choose to disregard the narrative by leaving after each stage to go work on something else.
By contrast, the difficulty curve of the Catacombs is gentle and gradual enough that a player can reasonably expect to ramp up to keep pace if they start at the appropriate level. (Caveat: Certain classes can have a very tough time with the final boss, who is immune to generic melee attacks.) Meanwhile, a group is probably bored by the relative lack of increased challenge.
Catacombs: The eight quests total for a base exp value of 5,814. On average, that means that each stage is worth about 725.
STK: The first stage, which, remember, is the only level 3 quest, has a base exp value of 1140. The three combine for just over 5,000 in base exp.
Borror0 noted that players perceive the Catacombs as having worse exp. In terms of base experience awards alone, that's not necessarily true - each individual step is worth less exp, but that is because each is also shorter than their STK counterparts.
However, there are two caveats. The first is that the long length of the STK quests make it easy to obtain the hefty bonuses for killing and smashing your way across the map. I think that the STK's may have more in the way of optional bonus objectives as well, so the real numbers may be closer. Then again, you could also argue that STK has an inflated exp number due to being higher level (and more difficult).
The second issue is one of perception; seeing the exp bar move by a few hundred is simply less impressive than seeing it jump by 4,000 in a single blow, even if the real exp/hour was no different.
A matter of perspective
In the end, both quests can be soloed, and both can be grouped. External factors, such as the availability of other competing quests in the same level range, and the general abundance of opportunities to kill undead in tombs across Eberron, also work against the Catacombs. Finally, group players may simply not be in need of an entire chain worth of level 3 content.
Even so, I wonder if the structural differences in the way these two quests are set up are a big part of their differing popularity. Personally, the ability to sign in, accomplish something meaningful, and sign out again in under 30 minutes is probably one of my favorite aspects of DDO. I can see how that same solo-friendly mechanic could get irritating for someone trying to keep a group together.