"I don't think there's a point.... It's not important to ask because it's [done]," she said, tearing up in her interview with NBC national correspondent Kate Snow.
Segments from the 25-year-old's interview appeared on the "Today" show on Thursday.
Williams sat for the interview to support the Challenged Athletes Foundation's endowment in her father's memory to provide sports adaptive prosthetics to those in need, she later said on Twitter. She'll be presenting the organization's Noble Award on Friday to honor her father and his triathlon teammates' humanitarian work.
When Snow brought up that people wanted to know why the Oscar-winning actor committed suicide last August, Williams couldn't offer an reason.
"Diseases are — until we find out exactly how they work — we don't have an explanation. So there's no one I can offer," she said.
Williams said that "there was an enormous outpouring of love from every corner of the world" when Williams died and she felt the support.
"It's not to say that people didn't know Dad. They knew a dad that he was proud of them knowing because he was an incredibly kind and incredibly caring man," she explained. "And he was also very private and very calm and very subdued. So the side of him that people know and love and that is attached to their childhoods [are] the characters that he had so much fun being. And that's what's important, and I do think that's what a lot of people will hold on to — that's not going anywhere."
"It's going to take a lot of work to allow myself to have the sort of fun, happy life that I had, but that's important. Anybody who has ever lost anyone works very hard to continue that memory in a positive way."
Dealing with losing loved ones goes "one step at a time," she said.
"The world keeps spinning," Williams said. "There's no point questioning it and no point blaming anyone for it, and there's no point blaming yourself or the world or whatever the case may be because it happened and you have to continue to move and you have to continue to live and manage."
Williams also said she commemorated her father by getting a hummingbird tattoo.
"Hummingbirds are fun and flighty and strange. It's hard to keep them in one place, and Dad was a bit like that. Keeping a conversation in one moment was impossible with him."