Why? Because she is incredibly sick!
Demi took to her social media to share the said news to her Lovatics, obviously feeling horrific for having to cancel the show.
"I write this through tears of sadness and frustration," she explained, going on to reveal that she's been "very, very sick" with swollen vocal chords. "If I go on tonight and injure my vocal chords any more, I could potentially have to cancel the rest of my tour."
She went on to apologise to her fans. "I am so, so, so sorry to disappoint any of my fans in London that have traveled hours to be here by plane or car, and to all of the fans that got these tickets for their birthday, holiday, or any other special occasion," Demi said. "It breaks my heart to let you down." Thankfully, Demi rescheduled her show for June 25, writing: "Thank you for understanding. I love you."
Thankfully her fans are super supportive of her, and care for her health.
"Get well soon honey that's what matters," one fan wrote, adding: "We care about you more than the show." Another commented, saying: "We are with you and your health comes first. We love you."
Demi also took to her Instagram story to explain the situation further.
"I can't do the show tonight because I have no voice," she says in the clip. "I'm very upset and really sad to let my fans down.... But I really can't go on, and I'm really sorry. We are rescheduling the show, I'm just very very sick right now, and hopefully I'll be able to perform at the O2 again very soon."