The Benedict Cumberbatch detective drama has proven to be a huge hit in the UK and US, but Stephenson - the BBC's controller of drama commissioning - told The Hollywood Reporter that the show's "tone" made it a risk for the corporation.
"In my opinion, if you take risks with stories and writers, you will actually drive big audiences," he said. "For example, the tone of Sherlock only works on the BBC in this country. The risk we took with that tone drove the success of it internationally."
He continued: "[Sherlock] is a massive show internationally. It is in hundreds of territories internationally, is a hit in America and has made Benedict a star.
"That came out of the same approach and attitude that goes into our other work. Being popular is absolutely at the heart of what the BBC does."
Sherlock co-creator Steven Moffat has previously admitted that he did not expect the show - which also stars Martin Freeman - to be a mainstream hit.
"We thought it'd get 4m [viewers] and an award from a Polish festival, something like that," he said in August.
Sherlock producer Sue Vertue confirmed last year that shooting on a third series had been pushed to March "for availability reasons" but insisted that the delay was "not expected to affect any likely TX [transmission] dates".
> Sherlock star Martin Freeman "in the dark" about third series
Watch Martin Freeman talk Sherlock's third series with Digital Spy below: