How do you know that those guys will be willing to face Brook?

We know Khan isn't.

Broner needs a catchweight with Porter, certain Brook couldn't agree to that. Will Broner be prepared to travel to England? Same goes for Bradley and Garcia - would Bradley be prepared to play second fiddle at this stage of his career? Is Garcia moving up? These guys have been on 7-figure purses, they wont be taking a paycut. Lots of intangibles.