As Stephen Colbert was setting up a bit on his final episode of “The Late Show” Thursday night, he inadvertently explained why his canceled CBS program became so unbelievably off-putting and tedious.

“We thought the best way to celebrate what we’ve done over the last 11 years is to do a regular episode where I come out here and talk about the national conversation,” Colbert said.

Are there any two less entertaining words in the lexicon than “national conversation”? Calculus lecture, perhaps? Rock tumbler, maybe?

The gag was that the fired host then got interrupted by a bunch of celeb pals including Bryan Cranston, Paul Rudd, Tim Meadows, Tig Notaro and Ryan Reynolds, all of whom were vying to be his last guest. Not exactly top-drawer stuff.

Nothing, save for a performance of “Hello, Goodbye” by the great Paul McCartney, was memorable. It was indeed a regular episode in that most of the jokes were clunkers.

But as CBS put the nail in the coffin, Stephen hit the nail on the head. His series could have 100% been titled “The Skewed National Conversation with Stephen Colbert” the whole time.

What was once a comedy talk show had obnoxiously careened into Rachel Maddow with musical guests and the same stylist.

Everybody — even his most devoted fans — knows this to be undeniably true.

Colbert largely avoided politics Thursday outside of some thinly veiled references to President Trump and a black hole sketch featuring astrophysicist Neil deGrasse Tyson, which turned the outer space anomaly into a metaphor for the forces that gave him the boot. Some more laugh-free filler.

Despite politics’ absence, sober, un-fun, slanted discourse is what defined Colbert’s unremarkable run “The Late Show” — especially his open antagonism toward Trump.

The small segment of the population who mystifyingly wanted that at 11:30 p.m., right before bed, pushed the program narrowly to No. 1 in the ratings. And thus Colbert came to see himself as a martyr for the cause and a strident warrior who holds truth to power. A Supreme Court Jester.

That kind of high-minded, grandiose thinking from a network comedian is delusional. It was a ratings grab and nothing more. I was reminded of the old Walter Winchell quote: “The way to become famous fast is to throw a brick at someone who is famous.”

Turning late night into an angry second hour of the evening news gave Colbert and the likeminded Jimmy Kimmel — both of whom only make headlines if the word “Trump” is also in them —  a boost. But the tactic has effectively rotted the institution’s foundation.

Thanks to “The Daily Show”’s snarky Gen X spawn, who increasingly make Jon Stewart look like John McCain, younger generations now believe that these shows that used to feature some of the most innovative comedy on TV (Letterman, Conan, Craig Ferguson) are where you go to watch a power-suited Nancy Pelosi take pot shots.

The podcasts and YouTube shows they’ve turned to instead are friendlier and funnier. Young, weird comics from Brooklyn are fresher, sharper and better.

Colbert’s acrid model was unsustainable. When Trump’s second term is over, those little lefty late-night audiences would have become microscopic. They definitely will for Kimmel. We’ve already seen it happen to the Washington Post and what was once MSNBC. As soon as their customers’ outrage diminished, they jumped ship en masse.

Meanwhile, the companies alienated everybody else.

Of course, the most iconic send-off to a late-night host was Johnny Carson’s final “Tonight Show” in 1992, when Bette Midler emotionally serenaded the beloved legend with “One More For My Baby.”

You felt like you were losing a family member or your best friend.

Despite an insufferable yearlong windup of sycophantic adulation and poor-me whining, Thursday’s finale didn’t feel like we lost anything.

Rather we gained a timeslot.