There are many reasons that a title is not licensed here in North America. It's viewed as difficult to sell; it has content which is acceptable in Japan but questionable here; the artist has demands the publisher cannot meet; the price tag the Japanese publisher asks is too high...
And then there's just that no one wants to license something 100 volumes long. Japan's classic, epic masterpieces can go on... and on... and on. If you license a 5-volume series and it doesn't sell, that's merely a poor return on investment. If you commit to a series with 50 volumes or more... it had better be able to print money.
So let's look at the longest series, by volume (according to the ever-reliable Wikipedia), and see what they are and why they're so beloved, along with why they would (or would not) do well over here.
1) Kochira Katsushika-ku Kameari Kōen-mae Hashutsujo, aka KochiKame. The big one, folks. This is 170 volumes long, and still running. It began in Weekly Shonen Jump in 1976, and likely will only end with the retirement of the author. The title translates to This Is The Police Station In Front Of Kameari Park In Katsushika Ward. I suspect, were it ever to appear here, it'd just be called KochiKame. KochiKame is a standard gag manga, revolving around a goofy middle-aged cop, Ryotsu. He's easily recognized by his distinctive eyebrows, which look like a McDonalds sign in the center of his head. (A recent volume of Gintama parodied his eyebrows, and he also appears in a cameo towards the end of Enies Lobby in One Piece.) The manga revolves around his police duties, his money-making schemes, and his overindulgences.
With its utter lack of romance, serious action, or car chases that go on for chapters, this is not exactly made for North America. If it ever were licensed here by Viz (it's a Jump title), I could see it being a best-of omnibus a la Oishinbo. It's fairly episodic. However, more likely it will remain Japan's most beloved cop unknown in these parts.
2) Golgo 13. Ha! Tricked you, this *was* licensed. Twice. However, both Viz licenses were for omnibus best-ofs, a very sensible way to go. Viz's was, in fact, incredibly well handled. The original, of course, debuted in the pages of Big Comic in 1969, and still comes out today, 154 volumes later (and counting). In case you were unaware, this is a very serious adult manga about a professional assassin. It too has no romance (bar Golgo 13's loveless conquests), but it certainly has plenty of action. The latest anime version is being released here soon; if that takes off, perhaps Viz might release a few more best-ofs.
3) Dokaben. This is the first one which is actually a trick volume question. Technically, this is four separate series, running 48, 17, 52 and 34+ volumes respectively. In actuality it's all the same cast and plot, so it totals up to 151 and counting. It's run on and off since 1972 in Akita Shoten's Weekly Shonen Champion. A Dokaben is a type of bento lunch, by the way. As for why it's not licensed, the reasons are endless. The art is 'old-looking' and caricaturish, and most of all, it's a baseball manga. Maybe if Cross Game sells as well as Naruto... nah, probably not even then. Still, it's well known enough to be namechecked in one of the Scott McCloud books.
4) Cooking Papa. A classic title from Weekly Morning, Kodansha's top-selling men's magazine, this has been running since 1984. About a salaryman who cooks, this is pretty much a classic food manga, a staple of many Japanese men's manga magazines. He's got a lantern jaw, and actually looks kind of like the square-jawed Batman of recent DC animated series. He is, of course, a sweetie pie in reality, who loves his wife and daughter and loves to cook for them. The recipes (and yes, they are included within) are simpler and more 'down home" than in the more exotic food mangas, but then this is meant to be basic comfort manga. I don't really see it getting licensed here, though, unless Kodansha USA want to do a best-of.
5) OK, technically 5 is Nijitte Monogatari, a manga that ran in Shogakukan's new magazine Weekly Post. It's 110 volumes, and I can't find any information on it. If anyone knows any, you can post in comments. :) So I'll go with Oishinbo, which... hey, has been licensed here in a best-of omnibus! From Shogakukan's Big Comic Spirits, this is of course the story of a grouchy food lover and reviewer, Yamaoka, and his relationships with both his estranged father Yuzan and his cute and sensible co-worker (and later wife) Yuko. But really, it's about great food, lovingly depicted. Like Golgo 13, Viz did a bang-up job with the 7 volumes it released here, and the series was much beloved by bloggers, if not so much actual buyers.
As you can see, none of these series are completely unlicensable. (Well, OK, maybe Dokaben is.) With the right selling point, and a relatively continuity-free series that can be dipped into at random, you can make this not only work but work well. I'd certainly buy a 7-volume best of KochiKame. How about the rest of you?