What could be the reason for Oprah absence at Madiba memorial?

Naomi Campbell and Charlize Theron attended Nelson Mandela's memorial service on Tuesday.
The U2 frontman, British supermodel and Monster actress joined political leaders from around the globe to pay tribute to the former South African president - who died last week aged 95 following a long illness - at a service in Johannesburg.
South Africa native Charlize - who first met the anti-apartheid campaigner in
2004 - was one of the first famous faces to arrive at the FNB Stadium in Johannesburg, and blew kisses as she was cheered by the waiting crowd.
Talk show host Oprah Winfrey had been due to attend the service, but reportedly failed to arrive.
Shortly after news of Mandela's death broke on Thursday night, Winfrey shared an emotional message on her Instagram account.
"One of the great honors of my life was to be invited to Nelson Mandela's home, spend private time and get to know him. He was everything you've ever heard and more – humble and unscathed by bitterness.
"And he always loved to tell a good joke. Being in his presence was like sitting with grace and majesty at the same time. He will always be my hero. His life was a gift to us all," she wrote adding a photo of her with the icon.
The reason for Winfrey's absence from the event is not yet known.