The Necromancer is Underpowered Reason 1: Everyone else that uses poison is more versatile

Any character in Diablo II can use poison from charms or equipment. The damage from charms and equipment alone isn't huge, though. Not everyone has poison skills, which can make poison a primary damage source. But of the classes that do have poison skills, all can put it to good use, at least against most monsters. The amazon, druid, assassin, and necromancer all get poison skill damage.

The amazon has Poison Javelin and Plague Javelin. Maxing both skills and using appropriate equipment give considerable poison damage. The druid in his werewolf form has Rabies and a little help from his Poison Creeper pet. The assassin has the incredibly powerful Venom skill. All of these are viable. The necromancer gets more poison skills than the others, with Poison Dagger, Poison Explosion, and Poison Nova, and his poison skills all synergize with each other. On top of that, the necromancer has the Lower Resist curse, letting the necromancer weaken enemies to his poison and even break the poison immunities of most monsters.

All of that would make the necromancer seem to be the best poison-user, not the worst. Other characters have less poison damage and can't weaken the poison resistance of monsters. The problem is that for any of the other characters, investing in poison can be part of a versatile build. For the amazon, poison skills lie in the spear and javelin tree. Maxing both poison skills leaves her with plenty of room to stack points in the other spear and javelin skills for lightning and physical damage. A werewolf druid can easily dump points into both Feral Rage and Fury (Fire Claws would probably be a bit of a stretch). An assassin can max Venom and go with pretty much whatever else she wants. So yes, the necromancer can get the most poison damage and can break some poison immunities. That's good for him and makes a "poisonmancer" one of the most viable options for killing monsters with a necromancer. But some monsters have such a high poison resistance that Lower Resist cannot break their immunities and some poison immune monsters cannot be cursed. This isn't much of a problem for other poison specialists, because they can easily destroy poison immune monsters with damage from other skills.

For my own poison necromancer, I addressed the issue by equipping the full Trang-Oul's set, giving me access to fire damage, and using Grief as my switch weapon. With the Lifetap to keep my health from dropping, I can wear down almost anything. But it's slow and clunky. It relies on a full set of elite equipment. Now, this certainly works. My poison necromancer is powerful. But I contend that I could get similar levels of power with other characters while using cheaper gear.

Lower Resist is essentially only available to a necromancer (there are items that can cast it, but they only provide low-level versions of it). In terms of sheer poison power, especially factoring in Lower Resist, the necromancer beats everyone. This might seem like an odd point in favor of my claim that the necromancer is completely underpowered, but I'm leading with poison because it's all downhill from here. The necromancer gets his greatest strength, the one thing he does better than everyone else, first. His other skills are weaker in comparison.

Most importantly, the necromancer has the the worst capacity for complementing poison damage with damage from other skills. Maxing all three poison skills requires 60 points. Then there are the prerequisites and the extra points into Lower Resist (I've put five points into the curse for my necomancer and let +skills do the rest, because upgrading the curse yields diminishing returns). That doesn't leave much to work with. The bone spells require even more points. A poison necromancer could put points into the summong tree and let pets do part of the killing, but the skeletons will be too weak unless some poison damage is sacrificed. You can have a poison necromancer with massive dagger/nova damage or you can have a summoner with powerful skeletons, but you can't really have both. And a poison/summon hybrid still runs into the problem that summoners have in general. More on that later, though.

This is why I use the word "versatile" in the title here. Other poison specialists can branch out into other skills and be effective with them. It's no problem at all for the assassin (Venom only takes 20 points) and the druid and amazon even have skills that could be one-point wonders like
Fury and Fend. The necromancer doesn't have those options. Poison is all he has.