The others had other things going for them as well to make them more than a five minute driveby. For starters I'm doubtful as to whether plaguefang is duoable. I can only really think of one combination that has a shot at doing it, and it includes pretty much 2 characters out of the ones on the server. Same thing with lavascale, though I'm imagining at least 6-10 characters on the server could be used to do it, as he's much less difficult. Fluxator? In a dungeon. The only one who really even comes close to fitting that was made that way is lenny. Also, plaguefang if you're not bound at the actual lake, is pretty much a comparable run to pretty much any dungeon zone tbh.
To explain, getting together at least another person to snipe a named, and splitting the money, is not even close to as beneficial in most cases as going yourself, to a dungeon, and hitting up a bunch of named in a row which have a chance of respawning for more loot in a very short period of time (as opposed to like 4 days). For starters we have the difficulty, then we have the relative prices of the gear, then count in the splitting versus not, and the time to get someone else on at the same time as well, versus invising to a bunch of named.
Rotlord really is the only one of those other than lenny that fits the 5minute driveby explanation, and unless you want a literal ton of frenzied worshippers kicking your teeth in, it's advised to take your time and take care of them beforehand. Lenny and rotlord also have quests attached. A much better rationale for making their loot nodrop.