Diaz was scheduled to appear at news conferences in Toronto and Las Vegas this week to promote the fight, but failed to make both. According to UFC president Dana White, the promotion booked the fighter several flights out of San Francisco, none of which he took.
With Diaz and his manager, Cesar Gracie, not returning his calls, White made the switch just prior to Wednesday's news conference in Las Vegas.
"Am I supposed to move forward with this fight, feeling confident this kid's going to show up on the night of the fight?" White said. "I'd rather pull the fight now than have him not show up to the fight."
Shortly after announcing the switch, White took a call from Gracie from the podium at the Mandalay Bay Events Center. After a brief conversation, White relayed that Gracie was as confused as anyone.
"He said, 'I apologize and I agree 100 percent with what you're doing,' " White said. "Nobody can find (Diaz). He thinks it's as weird as we all do."
When contacted by ESPN.com, Gracie said Diaz's actions were a "great disappointment." The manager said he even offered to drive Diaz to Las Vegas late Tuesday night but didn't hear back.
"A lot of people invested quite a deal in this fight," Gracie told ESPN.com. "The fact that Nick is now playing games and can't be a man and handle his responsibilities is not good."
White said he had not decided whether or not to cut Diaz from the UFC, or from Strikeforce, where he holds the 170-pound title. The two organizations are owned by the same parent company.
"I haven't had time to think about it," White said. "But I have to assume that, yeah, we'll let him go."
St. Pierre (22-2) appeared bewildered by the course of events. When asked to provide some type of guess at Diaz's actions, the champion couldn't come up with one.
"There's a lot playing in my head right now. Part of me thinks he's going to walk in right now and start calling me names," St. Pierre said. "For me, I work all my life for this opportunity. It's the opportunity to get out of the shadow. His life would change completely.
"To be honest, I don't like doing this. I would rather stay home and train. I despise this. I don't like the staredown and the 'click click' pictures, but I have to do it. And I do it every single time."
He did have favorable words for Condit (27-5), praising him as a "more dangerous" opponent than Diaz. Condit is riding a four-fight win streak that includes three finishes.
White did not hold back on his theory behind Diaz's absence, saying he's never seen anything like it in the 1,600 fights he's promoted.
"Nick Diaz obviously can't handle the pressure of a main event of this magnitude," he said. "This is crazy. This is beyond insane."
Tickets for the event went on sale Wednesday morning. Because the main event has changed, White said fans would have the right to a refund.
Brett Okamoto covers mixed martial arts for ESPN.com. Staff writer Josh Gross contributed to this story.