Forgive me for quoting George Lucas (who was himself paraphrasing someone else, I believe), but it seemed on point for this post -- "A movie is never finished, only abandoned."
That's my experience with sandbox campaigns too. Whether this is a problem or not depends on one's point of view. For me, it's neither good nor bad, but abandonment does, in my experience anyway, seem to be the end state of every sandbox campaign in which I've ever participated. Because there's no relentless drive toward a foreordained "climax," sandboxes just chug along happily until they run out of steam. Depending on the players and referee involved, this can take years or it can happen in mere weeks -- I've seen both happen.
Speaking as the current referee of a sandbox campaign, I'll admit that this can sometimes be a drawback. There are rarely any convenient "breakpoints" in a sandbox campaign. That makes it hard for me to put the the campaign "on hiatus," something I've contemplated from time to time. Now, that's not necessarily a terrible thing; the lack of breakpoints certainly ensured that the campaign survived my own occasional bouts of Gamer ADD. However, like most people, I appreciate closure when I can get it and just moving on from a campaign while still in media res never feels right.
Conversely, I have never successfully returned to a campaign that had an overarching story and that had ended upon the conclusion of that story. I've frequently wanted to do so, but, every time I've tried, it always felt a bit like I was gilding the lily. That is, if, at the end of the campaign, the characters defeated their long-time nemesis and saved the world, what more is left for them to do? Indeed, almost anything the characters might do threatened to cheapen their previous accomplishments by one-upping them, as seems to be the rule in such sequels (because, let's face it, why bother with a sequel if it isn't going to be more impressive than what came before?).
In the same vein, I have successfully restarted sandbox campaigns after they'd been abandoned. In fact, I've done it several times. The main difficulties are two: disinterest and forgetfulness. Once you stop playing a campaign, you can "lose the taste for it," if you know what I mean. A big part of what makes a campaign successful is regular play, which reinforces interest in continuing to play. Few things are more detrimental to a sandbox campaign than the lack of regular play, for without it, the campaign lacks cohesion and direction. Too great a gap between sessions also feeds forgetfulness, another bane of sandboxes. If either the referee or the players can't really recall what was going on in the campaign or why, attempting to return to it can be painful, like reading an Isaac Asimov sequel written decades after the original tale.
As I get older, I find I much prefer regular sandbox play to adventure paths and story-driven campaigns, but I also realize that my preference means that I am locking myself into playing a single game in a single setting for an extended period of time. I certainly don't mind that, but I have mused before that the relative lack of popularity for sandbox-style gaming is likely related to most gamers' desire to play lots of different RPGs rather than focusing on a single one for an extended period of time. When I was younger, we dealt with this issue by running several campaigns at once, but we had near-infinite free time in those days and it will be many, many years before I am similarly blessed again (if ever).