Just in case anyone doubted “The Walking Dead” could continue the ratings surge its fourth season experienced last year, the staggering numbers AMC posted Sunday night should put any skepticism to rest. If beating everything in broadcast and cable in the 18-49 demographic was starting to seem anticlimactic at this point, the drama series managed to top the Olympics, too.

Because the word “hit” has been overused to the point of utter meaninglessness in the TV industry, it’s easy to overlook that we are witnessing an honest-to-god phenomenon in our midst. But make no mistake: “Walking” has a freakishly huge audience. Which begs a simple question: why?

Stifle the urge to retort, “It’s the zombies, stupid”; “Walking” is actually about as improbable a megahit as the TV industry has ever produced.

To simply chalk up the success of “Walking” to the viability of the horror genre is to misunderstand the show. It’s never been some splatterpunk thrill ride like the “Saw” film franchise; there’s actually just as much an arthouse sensibility to “Walking” as there is to other AMC shows like “Mad Men” and “Breaking Bad.” And as last night’s episode indicated, the series doesn’t adhere to conventional storytelling. Then there’s the tone of “Walking,” which is so bleak it practically thumbs its nose at mass appeal. Throw in the fact the show hasn’t minted any true breakout stars and attracts zero awards buzz, and these gaudy ratings are even more inexplicable.

Forget about the fact that it’s cable’s highest rated series, averaging about 13 million total viewers per episode in the first half of the fourth season; its 6.6 ratings average in the all-important 18-49 demo exceeds everything on broadcast (besides NFL telecasts) as well. And unlike most TV crowdpleasers that inflate their numbers by skewing older, the “Walking” audience boasts a sprightly median age of 33.

Also keep in mind that “Walking” is just the kind of serialized drama that could do well enough in binge mode to see a significant enough chunk of audience wait days after its premiere date to watch. The fanbase for shows like NBC’s “The Blacklist” are experiencing this siphoning via VOD and DVR, or perhaps even many months later on Netflix and DVD. But “Walking” is true watercooler TV; people watch episodes immediately.

Maybe it’s a little late to be awestruck by a cable series beating broadcast; the ratings parity trend has been a long time coming. Series from TNT’s former hit “The Closer” to A&E’s more recent “Duck Dynasty” may not be doing as well as “Walking,” but well enough to prove that cable’s days as niche entertainment are over.

But there are about five other cable networks one would name before AMC came to mind as a channel capable of leading this trend. It’s been more successful attracting buzz than boffo ratings. And a standard-issue procedural drama like “Closer” and an unscripted sitcom like “Duck” are more likely to be crowdpleasers because they come from the same mold broadcasters have used to crank out their own popular fare.

Say what you want about “Walking,” but this is not a mold we’ve seen TV use too often to generate big-tent entertainment. Even if you misperceive the series as some mindless exercise in horror that panders to bloodthirsty palates, that’s traditionally been more of a hot ticket at movie theaters than in living rooms. You could also argue the body count on “Walking” is just as much an impediment as it is an invitation to large crowds given the segment of the audience that doesn’t have the stomach to watch it.

The “Walking” logline makes it seem like a slasher film: A hardy group of survivors of a viral outbreak escape bloodthirsty zombies. But that’s really more of a hook for what may have its share of fan-friendly action elements but is at its heart a multilayered, character-driven piece. “Walking” is actually a very thoughtful meditation on the difficulty of retaining one’s humanity amid dire circumstances. It plays more like a Holocaust drama than a horror pic.

What’s truly mystifying is that the show seemed to have reached its audience peak at a time when the storylines couldn’t be less broadly appealing. While the previous season’s faceoff with the character known as the Governor provided a pretty conventional villain character to root against, most of this season was focused on the ensemble fighting an invisible enemy: a virus that decimated their ranks. One of the show’s most difficult but heretofore lesser themes — the plight of children in this war zone — was thrust repeatedly to the forefront of the latest episodes in all its depressing, hair-tearing ethical implications.

And when they did turn their attention to the Governor, they brought him back in most unusual fashion with a morally ambiguous storyline that essentially humanized a character that had engaged in mass murder.

Most amazing of all was “Walking’s” audacious decision to bench the entire cast for two episodes to tell the story of the Governor — name another show that would dare do that. If anything, the sidelining was tacit acknowledgement that its lead actors aren’t exactly featured attractions on this series. You there, “Walking” fanatic: Can you name any of these people from memory? Andrew Lincoln? Norman Reedus??

It’s a talented cast, but they’ve been ignored by the Emmys et al. In all likelihood, that’s because series with sci-fi elements don’t tend to get much respect from awards voters. And with the exception of Jon Bernthal, whose character was killed off in season two, they don’t seem to be capitalizing much on their work in the outside projects that could help build their profiles. Still, how odd is it that the biggest show on television barely hangs its hat on the people who populate its cast — typically the most important ingredient to a series’ success.

No doubt there’s a cynical programming executive somewhere that looks at “Walking” and thinks, “Now if we could just work in a guest arc for (insert brand-name actor here), we could raise the show’s visibility even more!” It’s just one of many TV tropes that “Walking” has managed to eschew without sacrificing its appeal. Somehow the show has gotten this far without resorting to the usual tricks: the cheesy love triangle, the awful catchphrase, the curvaceous or studly character who somehow manages to look sexy in a post-apocalyptic war zone. There is nothing sexy about “Walking.” Ever.

It’s surprising that no network has tried a more commercial (and less depressing) version of “Walking” because AMC has left that space wide open (Amazon Studios came closest, developing a pilot based on the movie “Zombieland” before passing on the project).

Imagine what “Walking” would have been like had it been developed by a broadcast network. Think of how prominently, for instance, the race to find the cure or cause of the virus that created the zombies would figure into the narrative – -an aspect of “Walking” that goes entire seasons without being addressed. On ABC, they’d have the cure discovered by the end of the first season; because they’ve barely touched upon it on AMC, the show feels like it could go 10-plus seasons easily.

There’s a fascinating question critics should be answering: What is it about a show that is so relentlessly bleak that allows it to still resonate at such unexpected scale? What does it say about America? Of all the shows that manage to aggregate a mass audience at a time when a rapidly fragmenting audience is making it increasingly difficult to bring a sizable chunk of people together to share anything, “Walking Dead” is the show that threads that needle. And it’s the polar opposite of the escapist fare that typically serves as popular entertainment, a dystopian nightmare if there ever was one.

But “Walking” doesn’t get nearly as much respect as other series seen as more creatively substantive. While the press certainly don’t ignore “Walking,” they clearly don’t lavish the love that fellow AMC series engender. “Mad Men” or “Breaking Bad” somehow achieve far more cultural cachet and critical acclaim with nowhere near “Walking’s” average audience.

Critics can defend themselves by saying their job is to give disproportionate attention to the shows that are the best creatively even when they are not commercially successful. That’s all well and good, but it’s a shame if that mandate is fulfilled at the expense of failing to reckon with a true cultural phenomenon in progress.