"There's so many different explanations" for the decision, Neame said in a conference call with reporters to announce the sixth season would be the last. "I think our feeling is that it's good to quit while you're ahead."
The period production set in early 20th-century Britain highlights the "upstairs-downstairs" class divisions of the time and the impact of social change and world events in plots filled with human drama.
A recent episode depicted the growing women's suffragette movement, but the script also is heavily laced with deaths, marriages, love affairs, adultery and homosexuality.
It has become one of the biggest international hits ever for British television, watched from China and Sweden to Russia and South Korea, with viewership in America surpassing all expectations and setting records for U.S. public service broadcaster PBS.
Neame denied recent media speculation the show was being wound up so that its main writer and creator, Julian Fellowes, could go to work on other projects.
"I would not be inclined to try and keep the show alive without Julian, he is the creator of the show, he's written every episode, he's created all of those characters and it's just been a fantastic partnership," Neame said.
"But it really isn't the case that Julian has said, 'I want out, I want to do other things' and everyone else has been forced."
Neame declined to discuss details of the scripts for the sixth season but said it was well into production.
He said there were no plans for spinoffs, but he did not squelch rumors there may be a "Downton Abbey" movie.
"I can't confirm definitely it is going to happen...we will see," he said. "It's a very emotional day for all of the people who have been involved in making the show.
"It's been a big part of our lives and we will all miss it a lot but it's not the only production that we make," he said.
The cast has included some big names in the British theater and acting world, including Hugh Bonneville as Robert Crawley, Earl of Grantham, and Laura Carmichael as Lady Edith Crawley.
Maggie Smith, one of the doyennes of the British stage and film world, plays the Dowager Countess of Grantham.
Sets, including Highclere Castle in Berkshire, England, where the series takes place, have become huge draws for fans wanting to visit "Downton Abbey" locations.