(or "Loading Up Some More Railing Fodder")
When the Punisher first showed up in 1974, it was as part of a whole raft of changes to the way the superhero comic worked. The audience was beginning to skew older, attracted by Marvel's pop-art sensibilities and counter-culture street cred (it's no accident that the original Not Ready For Prime Time Players did Marvel comic-themed sketches on 'Saturday Night Live'.) At the same time, movies like 'Death Wish' (which came out after the Punisher made his first appearance, but was based on a 1972 novel) and lines of novels like 'The Executioner' series, by Don Pendleton, were bringing the costumed vigilante back to its roots as someone who dealt out harsh, unflinching justice on the enemies of society. The Punisher tapped into the "take no prisoners" zeitgeist perfectly--so perfectly that he became a major hit for Marvel almost against their will, soon developing into a character that had three ongoing series and rivaled Spider-Man for popularity.
In a way, the Punisher is a symbol of the way that superhero comics changed in the 80s and 90s. To many, he's a symbol of the cold, cruel, heartless nature of "modern" superheroes. Sure, he stops crime, but what kind of lesson is he teaching? What kind of morality does he espouse? Where are the higher ideals that human beings should try to live up to? Others see the opposite side. They see heroes like Batman and Superman as relics, unwilling to take the steps needed to really make people safe from Luthor and the Joker. By simply delivering these incorrigible criminals to jail time and time again, Batman and Superman actually enable them to continue their crime sprees, since it's obvious that the jails can't hold them. A superhero like the Punisher actually ends crime, if only on a criminal-by-criminal basis.
But, of course, that debate is irrelevant to us. We're looking at the Punisher from the point of view of a writer, and looking at it like that, being a grim and bloody vigilante might make society safer, but it makes the writer's job a lot harder.
Because one of the reasons that superheroes build up a Rogue's Gallery of supervillains is that it saves the writer from having to come up with a brand-new attention-getting antagonist for every story the hero goes through. Mike Baron, author of the "definitive" Punisher stories that fill 'The Essential Punisher Volume Two', tends to get around this by ripping his stories from the headlines (Volume Two alone contains a Charles Manson analogue, an obvious swipe of the Reverend Jim Jones, evil insider traders, and thugs who run a high school like their own personal kingdom. It's practically a catalog of 80s neuroses about society.) Garth Ennis tends to come up with inventively twisted and deformed mob bosses (which led, at one notable point, to the first time a superhero ever murdered a quadruple-amputee in the last issue of a storyline.) But everyone has the same problem. With one or two notable exceptions, like Jigsaw, people don't usually get a second go-round in a Punisher storyline.
This leads to two problems. One, obviously, is burnout. By the end of Mike Baron's run, he was clearly grasping for ideas (the Punisher getting plastic surgery, disguising himself as a black man, and hiding out with Luke Cage is a clear sign of "grasping for ideas"), and by then, readers and editors seemed burned out on the Punisher as well. Since every new villain wound up dead by the end of the story, it seemed like the book became a parade of faceless targets, and subsequent attempts to "shake up the formula" got further and further away from the core concept that had hooked readers. (Anyone remember the "supernatural assassin" Punisher series? Don't all speak up at once.)
The second is that the Punisher doesn't exist in a vacuum. He's a part of the Marvel Universe. Which means that he has to fit into a world where, in general, writers tend to keep their villains alive to fight another day. The "Punisher vs. the Kingpin" storyline that takes up most of the second half of Volume Two suffers in a big way from this; since readers can be reasonably sure that the Kingpin won't kill the Punisher, because his book is selling too good, and they can be reasonably sure that the Punisher won't kill the Kingpin, because the Spider-Man and Daredevil writers get a say in this, all that can really happen is a stalemate...which is, in fact, the end of the story. The Punisher can't make too big a dent in the criminal population of New York City, or his buddies will be out of a job.
In short, the reason heroes don't kill villains in comics isn't because they're noble, or because comics are for kids...it's because it's easier for everyone if they don't. The Punisher stands out as an exception, but he's yet to have a period of sustained popularity, because he's harder on his writers than most characters. Coming up with a good villain is hard, and the Punisher needs more good villains than most...because he chews through his supply quicker.