(or "Engines Within Engines Within Engines")
The 1990s were a very interesting time for the comic book industry. After decades of being dominated by the newsstand market, with its focus on single-issue storylines that could be easily picked up and understood by a casual reader, the medium was finally beginning to listen to the waves of long-term fans created by the direct market. These fans picked up every single issue, finding back issues to supplement their collections when needed or simply buying collections of their favorite book. They were tired of repeated exposition and short storylines that favored accessibility over depth. They wanted longer, more complex stories and they were vocal about it. (Depending on how you feel, this was either the best thing that ever happened to the industry or the beginning of its long, slow death march into cult obscurity, but that's a topic for another time.)
The change in focus from single-issue storylines to long-term plots had a major effect on the way storytelling engines were constructed. Many writers stuck with a fairly simplistic A/B/C structure, where they had a "main" A plot, a B plot that was usually foreshadowing the next A plot, and a C plot that ran underneath the main material for a longer payoff. (A sample Spider-Man story using this structure, for example, would have Spidey fighting the Vulture, with digressions to Doctor Octopus escaping, and occasional glimpses into the life of Aunt May as she worried about her health problems.) Others, like Chris Claremont on the X-Men, simply had multiple storylines going on at once of different lengths, counting on the obsessive nature of the fans to help them keep track of which mutant was amnesiac, which mutant was a fugitive, and which mutant was dead.
But John Byrne, in his creator-owned series 'Next Men', tried an entirely different tactic, one that would be far more familiar to modern audiences than to those of 1991. He came up with a single, overarching plot, one that was complex enough that it needed to be told in the form of several interlocking smaller stories. Each story, while relatively self-contained in and of itself, would advance the larger metaplot towards its ultimate conclusion.
Nowadays, this is a common trick for extended series; shows from 'Buffy' to 'Heroes' to 'Lost' did much the same thing. Each episode told a single story, while including plot points that rewarded careful and attentive viewing to lead to a larger conclusion...one that was made more meaningful by the emotional investment that the audience put into it over time. The last episode of 'Lost' wasn't just the end of the story, it was the culmination of six years of journeying along with the characters and finding out where they were going right along with them.
In the same way, 'Next Men' proceeds along with Jack, Nathan, Jasmine, Bethany and Danny. We see them at the start, confused and uncertain as their world is stripped away from them, and follow them along through a mystery that involves time travel, government conspiracies and eccentric billionaires. Each storyline functions in its own right, but Byrne is also telling a much bigger tale.
But it's not so much 'Lost' that provides the model we see here as 'Heroes'. In 'Heroes', the major, overarching plotline of the first season (Sylar and Peter's growing control of their powers and the ultimate question of whether or not New York will be destroyed) is intended to be only the first in a series of arcs that build to a yet greater purpose. Later seasons never quite followed up on that promise, and the series wound up being remembered less-than-fondly as a result.
Likewise, the saga collected in 'The Compleat Next Men' ends in a way that wraps up the whole plotline; the time-loop that we see the end of in '2112' and the beginning of in 'Book One' is filled in to the last detail, forming a seamless whole. And yet, the ending also tantalizes a future direction for the series...one that Byrne had to wait fifteen years to tell. Why did it take so long? (Apart, of course, from the 1995 collapse of the comics industry in the wake of the Marvel bankruptcy.)
Part of it lies in the storytelling engine, I think. Byrne created a large story that also served as a storytelling engine for generating its smaller, individual stories; but to fans of 'Next Men', those were only parts of a single whole. Many of them might not necessarily be fans of the overall engine that forms 'Next Men' nearly as much as they are of the overall story that forms the first 30 issues of the series, and they might not be willing to follow Byrne along in a new direction that forms a radical departure from what they've seen before...any more than they were willing to follow along when Peter became amnesiac, Nathan died and was replaced by a brainwashed Sylar, and Claire became a bisexual circus performer. (Or whatever happened...I'll admit, I get vague on the details of the latter seasons. Which is a factor in and of itself...the amount of planning time you have for a series is limited, which is why you use story-generating characters and concepts to begin with, and sometimes that second or third plotline suffers from the lack of prep time relative to your first, elaborately-planned idea.)
Ultimately, the long hiatus boils down to the fact that there's always plenty of competiton for people's time and attention (and yes, money.) It's hard to get people to follow along with a storytelling engine sometimes, but it's especially difficult when what they thought they were getting was just a single epic story.