I've been playing for a few decades now and campaigns have failed for several reasons. I'm going to avoid reasons revolving around everyone moving away, work schedules changing, or starting families.
Blue Planet: I tried running a
Blue Planet campaign around 2005 or so. The concept was that PCs were members of a University of Texas funded expedition to explore the planet Poseidon, and, if possible, to look into what happened to the previous expedition the university sent out which went missing. I gave everyone a monthly income based on their occupation with STEM PCs (doctors, engineers, etc., etc.) making more than other occupations. One player decided to make a private investigator, which was fine as security was needed and they were looking for missing people. I assigned the PC a monthly income that was at the midpoint for the party as a whole and that's when the problems started.
The player argued that the book says PIs make anywhere between 100-300 credits per day so his pay should be considerably higher. I pointed out that the book notes that PIs don't have steady employment so that 100-300 per day doesn't actually reflect their monthly average. With this job you're in the employ of an organization and are getting a steady paycheck and your character is actually making a pretty good living. He was like a dog with a bone and just wouldn't let it go finally saying, "I don't know why my character would even leave Earth for Poseidon." I told him it was his responsibility to come up with a reason why his character would leave Earth but at the end of the day it soured me to the campaign and I decided not to run it. It's the only campaign I've had that failed after character generation but before game play.
Curse of Strand 5e. The DM tried really hard. He had props and cool models and a perfect Eastern Europe accent. We just didn't buy into it. We stopped after two sessions.
Curse of Strahd: So I had a campaign come to an unceremonious end as well. I'll be as general as possible as not to spoil anything. The PCs were exploring an abandoned area when they figure out they were being spied upon and the PCs first reaction was to set upon this person and kill them. It left a bad tastes in my mouth and I ended the campaign stating that they all became evil and are lost in Ravenloft. I just didn't want to deal with that kind of game play.
I never plan out a campaign, I just start with a concept.
Savage Worlds: While I don't meticulously plan out campaigns, I do know what the bad guys are going to do unless the PCs interfere with their plans in some way. I do have a particular concept in mind though. I was using
Savage Worlds to run a Teenage Mutant Ninja Turtles game set about 50 years in the future. The PCs were mutants and their master was Raphael (Leonardo and Michelangelo were dead and Donatello was in another dimension). I was basing the game more off the comic book so I expected it to be somewhat violent but one of the basic goals of the PCs was to advance mutant rights. On the second or third session the PCs were participating in an illegal mutant fighting ring when the police show up to put a stop to it. The PCs proceeded to kill several police officers. Since this was all caught on camera, news outlets internationally had footage of the PCs killing police officers and any hope of them helping to advance mutant rights was right out the window. Within two sessions I had them escaping to another dimension and we went on to play something else. We weren't all on the same page so far as the campaign concept went.