(or "Because You Demanded It!...No, Really, That's The Only Reason")
It's never been any great secret that Wolverine, the X-Men's resident hardcase anti-hero, sells comics; you can pretty much take it as read that a big part of the success of the All New, All Different X-Men of the 70s and 80s came from fans of the Canadian mutant. But it wasn't until he got his first limited series, and the sales returns for it came in, that Marvel understood just how popular Wolverine was. From that point on, a Wolverine regular series became absolutely inevitable.
Which was, in a lot of ways, a problem. Because Wolverine as of 1988, when he first got his regular spin-off series, did not have a storytelling engine. He had no backstory (apart from a few mentions of time spent in Canada and Japan), and no intention of being given one--too many people behind the scenes at Marvel thought that an origin would diminish the mystery and intrigue of the character. He had no supporting characters who weren't already being featured in the X-Men (and using them would simply turn the book into another X-Men title), and as the X-Men were currently hiding out in Australia and believed to be dead, he couldn't exactly show up elsewhere and accumulate a supporting cast. He had no compelling reason to be on his own or agenda to pursue while he was having solo adventures; really, it was unclear what motivated Wolverine, besides the love of fights, beer, and redheads. All the things writers use to generate story ideas for their character, Wolverine did not have.
But he was very popular, and his name on the cover sold comic books. So he got a series anyway.
It's interesting to read the early issues of Wolverine, because Chris Claremont is practically running down a checklist of the elements of a storytelling engine when he creates Wolverine's new stomping grounds of Madripoor. Setting? Hong Kong mixed with Bangkok, filled with criminals and noirish characters, check. Supporting cast? Jessica Drew, the former Spider-Woman (as always, when in urgent need of a quick supporting cast, Claremont rings in characters from cancelled titles he's written in the past.) Several other noirish characters that run the bar Wolverine owns a half-interest in, check. Rationale? Wolverine is there in his guise as "Patch", checking in occasionally on Tyger Tiger, a half-brainwashed half-ethical crimelord that he feels personally responsible for (since he, along with the X-Men, aborted the brainwashing procedure half-way through.) He teleports there from Australia, using the services of X-Men ally Gateway, whenever he needs to participate in a thrilling solo adventure.
In other words, Wolverine is literally commuting to his storytelling engine every month. That's how difficult it is to create one for him. The X-Men is such a continuity-tight soap opera every month, and one that's so dependent on Wolverine (since he is, after all, the book's primary sales draw), that it's virtually impossible to set up the conditions that help a writer to generate independent stories. Any Wolverine solo book is "solo" in name only; he's as intimately tied to the X-Men as Professor X.
Eventually, long-time Wolverine scribe Larry Hama gave up on the Madripoor element and did what Claremont had fought against; the book became an adjunct of the X-titles, featuring numerous guest stars and a continuous soap opera following Wolverine's attempts to discover his true origins (a soap opera that went on behind the scenes at Marvel as well, since there were still a lot of people that thought the mystery was part of Wolverine's sales draw. The compromise solution involved endless false trails, red herrings, and matryoshka-like mysteries within mysteries.) In many ways, this describes the Wolverine solo book circa 2007, as well.
And yet, it sells. Wolverine seems to be an evergreen character for Marvel, someone whose popularity continues to sustain a series no matter what happens. In fact, a Wolverine movie is on the horizon...although, for some reason, they had a hard time coming up with a script.