When the cast of 'Project Runway All Stars' was announced, a few big names were surprisingly missing -- host and judge Heidi Klum, mentor Tim Gunn, and judges Michael Kors and Nina Garcia. Instead, supermodel Angela Lindvall will host the competition and Marie Claire editor-in-chief Joanna Coles will play mentor.

Fans have been wondering how 'Project Runway' can make it work without its big names. But now, we finally have an answer as to why Tim and co. didn't sign on for 'All Stars.'

"'Project Runway All Stars' overlapped with the taping of 'Project Runway' Season 9," Gunn told AOL TV, while promoting his partnership with Weight Watchers. "Someone actually thought that Heidi, Nina, Michael and I could run back-and-forth, from one place to the next, and I said, 'Look, I'm not speaking for Heidi or Michael or Nina because I can't. But I can speak for me. When I'm doing 'Project Runway,' I feel as though I'm overseeing an emergency room and that anything can happen. You want me to oversee two emergency rooms? That would put both shows at risk. That cannot happen."

"It may be a failing of mine, but when I'm with something, I'm there 150 percent," Gunn continued. "I couldn't possibly have a brain that's going back and forth between those two workrooms of designers. I was already totally committed to the Season 9 designers, and I couldn't disappoint them or fail them."

But the fashion expert, and co-host of the new ABC daytime series 'The Revolution,' thinks the all stars will be just fine without his help.

"I'm eager to see what it's like," Gunn said. "I think that there are going to be some incredible challenges. I would expect nothing less from an 'All Stars' season, but I have absolutely no idea. I don't even know when it's going to air!"

However, Gunn does admit that there are some 'All Star' contestants he'd rather not work with again.

"There were a few people I was happy that I didn't have to interact with."

'Project Runway All Stars' is hitting the runway Thurs., Jan 5. at 9PM ET on Lifetime.