She blew it. Big time.

In doing so, Oprah Winfrey revealed why even her appearances on her cable network OWN haven’t had a tiny fraction of the influence on American culture of her syndicated afternoon talk show. And that’s been true despite the high-octane pizazz of some of her “gets” and the moderate amount of celebrity news forthcoming from her sessions.

On a Sunday evening even more bursting with “appointment television” than Sunday usually is, Oprah’s interview of David Letterman on OWN far outpointed every neon-lit attraction on the Sunday strip: a new season of “Downton Abbey,” with Shirley MacLaine bringing American brashness to the “Upstairs, Downstairs” permafrost; Michael J. Fox on a new episode of “The Good Wife” on CBS; and a new episode of ABC’s “Revenge,” so that those of us who decided last season that it was one of TV’s guiltiest pleasures could figure out for ourselves whether it has now reached that pitiable stage where its writers are faced with devising new plots for a show they never thought would survive.

The way I looked at it, Oprah and Dave took top spot on the marquee. She’d landed – for a full hour – one of the toughest “gets” in the TV schmooze game, even now when late-life fatherhood and near-universal reverence have mellowed Letterman to the point where he’s no longer in stark terror of exposing the reptile farm that lives underneath the floorboards.

With the Kennedy Center Honors elevating Letterman – justly – to a cultural position none of his old elbow-flinging Comedy Store cronies could begin to approach, the boy has gone out there and met some proven interlocutors – most notably Charlie Rose on CBS’s morning news ramble because he couldn’t avoid it (CBS was running the Kennedy Center shebang the following week) and Oprah, who suckered him in by doing the interview in the David Letterman Building of the media program at his Indiana alma mater, Ball State University.

Letterman has previously “done” Oprah and Barbara Walters, but even TV’s version of prime celebrity interviewers haven’t come close, I submit, to mining what’s there beneath the jokes.

And here Oprah failed again. She alone had him for a whole hour – and, I’ll bet if she’d pushed it for her new network, a second hour besides. How I wish Charlie Rose had won the big-time, extended access Letterman sweepstakes instead.

The big “news” – such as it was – to come out of Oprah’s Letterman sit-down was his protracted six-month bout with depression, a “sinkhole,” he said, where “you can’t stand looking at the sunlight” and “you’re shaking, you’re shivering.”

And yet, said Oprah, you managed to do your nightly show. Sure, said Dave, he just pushed through it. End of subject.

Sorry. Not for me it’s not. There was a good 10 or 15 minute chat beyond that. How in God’s name does a man with such reputedly crippling depression get through an hour of one of TV’s most heavily scrutinized talk shows, night after night? How many drugs did he have to take? Who were the staff members who pulled his posterior up out of the abyss every night? What did they do to keep him functional? Did he ever stop functioning altogether? Are, in fact, all comedians a bit more subject to being bipolar than, say, plumbers?

I would think, frankly, that a man so indebted to what is clearly a singular support staff would want their names out there to share some of the Carson-plus limelight he’s now enjoying. I’d think Letterman himself would want fellow sufferers to know a little more about how one of the disease’s highest-profile sufferers kept on going.

But no. Oprah skated right over the top of it, even though Letterman seemed to be warily inviting her in for a deeper and scarier look.

Even worse was an incredibly ungainly – indeed stupid – Winfrey interruption when he started talking about his “sex scandal.”

It’s entirely possible, let’s admit, that she was adhering to a prior deal over just how deeply he’d let her go into his worst public ignominy – the blackmail over his affair with show assistant Stephanie Birkitt by her former boyfriend (later tried and convicted for it).

Even there, Letterman was giving her openings to go far deeper.

She skated by them in full “lah-di-dah, I’m interviewing Dave” mode.

I couldn’t help wondering if the absence of Winfrey’s old BFF Gayle King – who’s off at CBS mornings with Charlie Rose – didn’t reveal to us how limited an interviewer Oprah Winfrey really is, without proper preparation. Now, maybe, we know who the brains in Oprah’s outfit really was.

Here, just for the record, are a few things I want to know:

• Letterman wasn’t just having an affair with Birkitt, he was featuring her prominently – and indeed charmingly – in off-the-wall segments on the show. How did that sit with other female staff members?

• How badly was Birkitt’s life mangled by the process? She’s not the eternal symbol Monica Lewinsky is, but for a very bright and ambitious young woman, that couldn’t have been easy.

• His claim that his behavior revealed him to be not quite the “good person” he thought he was is so over-the-top, touchy-feely Oprah psychobabble that it avoided completely the obvious: how long he preserved the early life sexual morality of Comedy Club megastar (in clubs full of waitresses and audience members eager to hook up) within a settled married life where a young son had (in his fine words) caused a “window opening in your heart.”

He sounded like a man making good, solid Oprah noises. I’m guessing John Updike’s “jubilant, horrible truth” sounds a bit different.

• Jay Leno, he said, “is the funniest guy I’ve ever known” but also “the most insecure person I’ve ever known.” Follow-up, please – beyond Leno hiding in closets while his fate was discussed. Lots of it. But no.

• Johnny Carson had a “love-hate relationship with alcohol” and after a fair amount of vodka with Letterman at dinner, Carson’s young acolyte would encounter a nasty “edge” from his revered idol. How so? Examples please? Nope. Oprah wasn’t interested.

So why, you might wonder, does all this matter?

Because Letterman, as the Kennedy Center Honors indicated, has a unique place in TV history.

It was he, more than anyone else, who gave America a valiant and human Manhattan face after 9/11. He led the way out of darkness in a way that not even Carson ever had.

But then, had any other TV show also ever been simultaneously so open and so gracefully emotional about a star’s open-heart surgery? No. He’d shown a country back then what it’s like to survive the most intimate possible conversation with mortality.

He’s a man with a track record of emerging triumphant from some significant depths.

On Sunday night with Oprah Winfrey, he didn’t have to.