When continuing the campaign seems to require more motivation than just scrapping it. The root cause may be scheduling, or distraction by something new, or whatever, but they end when the group, and especially the GM, just decides that it isn't worth the effort anymore.
I feel like the solution is to specifically focus on modestly-sized campaigns with clear goals, and no expectation of the campaign lingering when you accomplish those.
Really, the question shouldn't be limited to campaigns. The same phenomena happens to TV shows, for instance, or book series, or movie franchises. Why do they fizzle out? Because people try to stretch them without any clear direction long after the interest in it starts to fade. You either need a clear direction and some discipline to stay on task, or you need to have limited scope and be willing to be done when you've accomplished your scope. Can you imagine if the X-files had always been geared towards a satisfying conclusion after 4-5 seasons or so instead of continuing into it's 11th or 12th or whatever the last season was, when the audience had long been wondering what the point was anymore? It's really the exact same phenomena.
And with campaigns, maybe there is a clear goal, but most published campaigns, at least, have a very meandering route to get there, which means lack of discipline on the main message. I think shorter, more disciplined campaigns would tend to get finished at a much higher rate than what we currently see.
Then, if it was popular with the group, you can discuss if you want to do a sequel or follow-up, or just start over with something else, rather than try to keep it open ended into perpetuity. That's the model that seems designed to fail more often than not.
