I think most GMs have them: campaigns we prepped for, recruited players for, maybe even had a session or two for, but then it just collapsed. What's yours?
The one that I think of when I think "failed campaigns" was this way overly ambitious Exalted 1E campaign that was straight up a retelling of the seminal 80s run of X-Men (but with a tough of the movies). I was writing for Exalted at the time so I was super confident. Character creation went great, with people choosing and building cool Exalted versions of key X-Men. I had big plans.
We played one session and the weight of the Exalted system was a hurdle too high. We never played again.
I guess personally I'd draw a distinction between "failed" campaigns, and "experiments that didn't work out". For me (and I'm not saying anyone else should be similar), I don't actually feel bad about a campaign being abandoned unless I invested a ton of effort on prepping for it, and/or the players invested a lot of effort learning systems and generating PCs, or we got part-way into it, and were enjoying it, but had to stop for some reason.
Personally I think of a few:
1) Champions: The New Millennium - Everyone was hyped about this, because we didn't like HERO/Champions, but we did like Interlock, and the FUZION version of Champions promised to combine the two. I worked up quite a significant plot (well, for me for a superhero setting). All the players came up with complex original-conception (not rip-off) PCs who they actually wanted to play. We did some pre-sessions with 1-2 PCs which went well.
Then we did the first real session and after like 20 minutes of investigation etc. got to a big set-piece fight which would sort of set up the campaign, and hopefully let everyone show off their powers and so on. I expected it to take an hour or two, max. Well over five hours later we finally finished the fight. Turns out C:TNE had the absolute worst aspects of both HERO and Interlock, not the best. People liked their character but no-one wanted to play that system further, so we were done.
2) WEG Star Wars - The Darkstryder campaign - This was actually not that long after the above incident. I was playing, not DMing (my brother was the DM). We all had three PCs (two pregen, one created) in a sort of Ars Magica-ish way (I dunno if that's normal for Darkstryder but I think it is), and we absolutely loved it. We played about three sessions, and it was like were in Deep Space 9 or something, totally amazing. Then the DM was flummoxed by something in the campaign, and he was like "Remember how I said someone should play the captain, and none of you wanted to?" and we're like uh-oh, and "Yes, does it matter?" and he's like "We can't continue because no-one is playing the captain". So of course literally all of us offered, but he was having none of it, and just was like "Gah this is too hard" and gave up. We've still never managed to get out of him exactly what the issue was - I keep meaning to read through the campaign and see if I can work it out. Very disappointing because it was so great.
3) CoC - Orient Express - My brother DMing again - It was all going too well. We got past
Baba Yaga and stuff and then... a vampire/tiger/vampire just killed all of us on the train and we had no idea it was coming for us, no idea how to defeat it with the resources we had and so on. We did have secondary characters ready (this is CoC, after all!) but the TPK quite late (or so it seemed) in the campaign, of easily our longest-surviving and most successful group of CoC characters just really took the wind out of us and we first delayed continuing to play something else (WoD I think) and then gave up.
4) Alternity/Dark*Matter - I had a whole campaign set up, and had tweaked Dark*Matter so it was less "right-wing conspiracies are true!" (let's not even talk about the Amazons of the Gynarchy people - if you really need to know an incredibly detailed breakdown/takedown of Dark*Matter is
here - I was astonished someone else not only spotted all this but wrote about it) and was very pleased with myself, it involved the then-recent Iraq war and ancient Babylonian evils unleashed on the world and so on. We got a little way in but the system... ugh... it was just not fun to play, and really didn't work for us. I still sometimes think about redoing it in a different system but I feel like it'd need a lot of updating.
5) Shadowrun 5E - I created a campaign based on where my wife is from - it had a name, a website and everything (The Shadows of Michiana) - I constructed a whole Shadowrun version of the area, with all sorts of unique features and plots (and it was also near Bugtown/Chicago for extra fun), and ran it for my wife and a couple of friends. They had really cool PCs too, not ones I'd expected. And this failed because oh my god is Shadowrun 5E an unrewarding mess to write for. Not are the rules kind of terrible (probably should just have used 2E or 3E), but they're very heavy and clunky on the back end, and require tons of prep, way more than I expected (I was anticipating WoD levels or SR 2E - the only version I ran significantly - and I might have had rose-tinted specs about that). We ended up playing a PbtA Star Wars campaign instead - which actually did go really well even if the players got way closer to just straight up murdering Mara Jade than I expected! Like, no, I have plans for her!
Looking at that it seems like system is the big killer for us, for actually making us leave and not come back intentionally.