NEW YORK (Reuters Life!) – It took months of angst for Conan O'Brien to get his own late-night chat show and if TV viewers are now wondering why it looks like all others, it's because that's what Americans like, industry watchers said.

O'Brien's new show 'Conan' premiered last week on cable channel TBS, marking his return to US television after an acrimonious departure from NBC's 'The Tonight Show' in January when he was replaced with that programme's former star, comedian Jay Leno.

But the 47-year-old known for his coiffed, red hair and off-beat humour didn't alter the basic chat show format – he wore a suit and tie, did a monologue before a studio audience, had a house band and musical guest, and sat at a desk talking to actors in a show that featured some light comedy skits.

"In television, things tend to change very, very slowly," said Richard Wald, media and society professor at New York's Columbia University.

"All of the best formulas are there because they have proven to be popular. It's like American beers – they all tend to taste like Budweiser because it's very popular. In television too, everybody seeks to find the centre of taste."

Wald said chat shows are not the only staid form on network TV. The evening news is virtually unchanged in 50 years, early morning shows share similar formats regardless and even entertainment programmes, from police procedural programmes to sitcoms, have stayed mostly unaltered for decades.

Bill Carter, author of the new book 'The War for Late Night' which recounts the battle between Leno and O'Brien, said efforts by others to shake up late night TV failed.

"There is something in the DNA of that format that people accept and they do not want it shaken up," said Carter.

Indeed, the format has been unchanged since the days of Johnny Carson, who began hosting 'The Tonight Show' in 1962 and quickly became the star of the show. In earlier programmes such as 'The Ed Sullivan Show', the host announced guests and entertainers in a variety show format.

These days, late-night talk show hosts from David Letterman to Jimmy Fallon follow Carson's format with few diversions.

"Every time someone has tried a different set up, whether its a different monologue or getting rid of the desk and sofa and just having two chairs, people have been uncomfortable with it and then gone back to the original way," Carter said.

For example, ABC's late-night host Jimmy Kimmel tried his opening monologue sitting behind his desk wearing a suit with no tie but quickly reverted to a stand-up monologue, including wearing a tie after audiences did not like it.

Carter said the problem now for late-night television is how to sustain the format with a dwindling audience. With their large bands, teams of writers, segment producers and costume departments, such shows are expensive to produce.

Leno drew six million viewers a few years ago, but his show has averaged 3.6 million total viewers this season. 'Conan' drew almost 4.2 million viewers on his premiere but his audience dipped to 2.8 million a day later and further slipped to 2.7 million by Wednesday.