The History Channel was originally scheduled to run the eight-part series but announced last week it was not a good fit for its fact-based slate of programming.
Starz and FX Pass subsequently said they were not interested in airing the miniseries either.
Showtime followed suit Wednesday.
"Although 'The Kennedys' is well-produced, well-acted and a quality piece of work, it doesn't fit the Showtime programming brand," a representative for the network told The Hollywood Reporter in a statement.
The premium cable network has not aired a miniseries since 2004 when it ran "The Reagans," the entertainment industry trade newspaper noted.
"The Kennedys" is still set to air in Canada March 6 and will be broadcast internationally on dates to be announced.
Producers of the miniseries drew flak in February 2010 when liberal filmmaker Robert Greenwald, after reading the script, called the project "political character assassination" and "a cheap soap opera of the worst kind."