The biggest reason we don't finish something is because we always want to try out the newest book, and sometimes find out that it's not that great. Sometimes we keep going anyhow (and I or whoever is DMing bangs it into shape). Occasionally we quit and move on to something else.
Maybe a topic for a new thread, but how do you know when its time to ditch a campaign and go on to the next new thing? When is it too limited, too broken, too uninteresting to be able to (or worth the effort to) bang it into shape?
As a DM I have only ditched one campaign since 2014. But it was because my players asked me to run Mage the Ascension campaign and I just wasn't groking or enjoying the system. Even more, it was just two much work. It was second game I was running in addition to my D&D campaign and I just didn't have time to prep and run too campaigns. I ditched the MtA campaign because it was easier and more enjoyable for me to run my D&D campaign.
I think because of my lack of time, I am careful what campaign I commit myself to. I may jump in and try a new system or adventure for a one shot, but I go into a campaign expecting to run it for a year or more. Months before I wrap up a current campaign, I start looking at game systems and adventures for my next. As I find a few that look interesting, I'll float a few options to the players, if they are interested, I'll dig deeper. When I was wrapping up my D&D 5e Rappan Athuk campaign, I came close to running DCC Dying Earth, but once I started digging deeper, I realized the lack of VTT support for the Dying Earth specific rules would make it a heavy lift for me to run remotely.
Also, other than Curse of Strahd, my campaigns are generally set in a world where the adventure path can be ditched without ditching the "campaign." If my players had tired of Rappan Athuk, I had lots of material where they could have gone off to other adventures in the Lost Lands setting. If they had done that, would that count as ditching the campaign? For me, it doesn't feel like it would be. It is still one story of one party, using the same system, in the same larger setting.
Yet, if my players had decided that they were not into Curse of Strahd when I was running it and I came up with someway for them to get their characters out of Barovia and then ran them through another WotC adventure book, it would feel like I was ditching the campaign. Which feels like I'm contradicting what I wrote in the prior paragraph.