The British screenwriter of Mandela: Long Walk to Freedom says Steve McQueen's Oscar-winning race drama 12 Years a Slave "sucked up all the guilt about black people that was available" during this year's awards season.

William Nicholson's controversial explanation for the relative failure of his biopic of the life of Nelson Mandela was delivered at the Hay festival at the weekend. He also told an audience that the South African freedom fighter's death shortly before the film's release had hindered its success and claimed to have invented most of Mandela's speeches in the film because the real-life equivalents were "boring".

Long Walk to Freedom, based on Mandela's autobiography of the same name, picked up just a single nomination for best original song at this year's Oscars following lukewarm reviews. Meanwhile, McQueen's harrowing tale won best film, best adapted screenplay and best supporting actress, as well as picking up nominations in six other categories and receiving almost universal acclaim from critics.

Long Walk to Freedom writer William Nicholson. Photograph: Sutton-Hibbert / Rex Features

Nicholson, 66, who received previous Oscar nominations for Gladiator and Shadowlands, said he was disappointed not to repeat the feat on the Mandela biopic. "I think it worked superbly," he told an audience at Hay. "I'm incredibly proud of this film. Unfortunately it didn't get the kind of acclaim that I wanted. It didn't get Oscars."

Continued Nicholson: "[Americans] were so exhausted feeling guilty about slavery that I don't think there was much left over to be nice about our film. So our film didn't do as well as we'd hoped, which was a bit heartbreaking.

"Mandela died as I was in the royal premiere with Will and Kate," he added. "We were deluged with Mandela stuff and after a week we all thought, please take it away, we've heard enough about Mandela."

Of the former South African's speeches, Nicholson said: "All but one ... were made up by me because his own are so boring. I know it sounds outrageous to say a thing like that, but when he came out of prison he made a speech and, God, you fell asleep."