Because they're fighting in October instead.
"[You] wanted it! [YOU] GOT IT!!! GSP vs Diaz Oct 29th UFC 137 at Mandalay Bay in Las Vegas," White Tweeted on Wednesday afternoon.
The bout had been heavily discussed over the last month, with White saying in a number of interviews that it wasn't close to a done deal and that there were still a lot of hurdles left to clear in order for the fight to take place.
Consider them cleared.
St. Pierre will make his seventh straight title defense in the UFC after dispatching of Diaz's Cesar Gracie teammate Jake Shields at UFC 129 in Toronto.
Diaz steps into by far the biggest fight of his career, and returns to the UFC on a 10 fight winning streak and off the biggest win he's had in the last several years over Paul Daley in April.
Neither fighter has lost since 2007.
Penick's Analysis: This is huge. There won't be an Ultimate Fighter lead in, but this fight didn't require it. As the one of the biggest draws in Strikeforce, the holdups were all likely centered on Showtime not wanting to have that draw be at risk of a loss elsewhere. But it doesn't matter what the holdups were, because the fight is happening, it's coming to Las Vegas, and it's going to be huge.
[Georges St. Pierre art by Cory Gould (c) MMATorch.com]