Some guy over on Matt's site asks why The Wire doesn't get better ratings if it's so damned good.
Here's why: it's ridiculously uncompromising. The street argot is very real and somewhat difficult to follow at times, the plot is something you really need to watch from the first episode, the cast is huge, bad or damaged characters aren't always (or even often) redeemed in a TV-friendly fashion, there's no happy endings, and the whole thing is intended to track the slow decline of American institutions. So it's a little bleak.
It doesn't help that it's a novel that happens to be on television, rather than a TV show. There are few single-episode plots. Most arcs cover several episodes, many cover entire seasons, and the important ones cover the entire show. You probably won't get The Wire if you only watch one episode. David Simon knows this very, very well. He just doesn't care.
It's also very, very black. Considering the dearth of good roles for black men (it's even worse for black women), it's richly ironic that the best television show ever made has a majority black cast. Not just the dealers, but most of the cops and many of the politicians. Heck, a lot of them are locals! The cast is one of the main reasons why the show is so good, with standout performances on all sides, but David Simon believes quite strongly that it's going to be a bit alienating to white audiences, and I don't think he's wrong on that. Black drug dealers are the "Other" to many Americans. They sure as hell don't want to sympathize with them!
Compare that to, say, The Sopranos, which is a far safer and more accessible show, and you can see why The Wire doesn't get Sopranos numbers. That said, though, I'm certain that The Wire is going to have an incredibly long tail. It's a show tailor-made for DVD or digital downloads, so much so that watching it on DVD is almost a different experience thanks to the "television novel" aspect coming into full effect. I think that's one of the reasons it was so vigorously pirated last year; the leaked episodes let you watch the whole thing in order in several sittings instead of having to wait a week, and I can attest (having the entire thing to date on DVD) that that is the only way to watch the show.
Hell, it might have a long tail just because it's heralding the death of the "TV show".