Brock is getting the fight because most people don't give a damn about Nog...he's boring. Brock, on the other hand, everyone cares about; some want him to win, some want to see him get destroyed, but damn near everyone CARES one way or the other. And when you care, you buy the PPV. Dana White is no dummy.
It's win-win for Dana. If Brock loses, he's just not quite ready for primetime yet, and Randy goes on to have one more big money fight before his last ever huge money fight against Fedor (this WILL happen, too much money to be made for it not to happen). If Brock wins, he's legitimized as a top heavyweight and draws sick money for many fight to come...and the Fedor-Randy fight is just as big a deal as it ever was.
That's what promoting is all about.