Fedor is still not that widely known. The hardcore fans do their best to get his name out there but Strikeforce and M-1 have done a terrible job at promoting him. When there is a UFC card, how often do you hear about it? Don't you see it plastered all over everything everywhere? Now think back to the last few Fedor fights. How much advertising did you see for them? It was minimal at best. Granted if he came to the UFC, they would get behind them with their marketing team and market the shit out of him.
Brock already had a huge following because of the WWE. He didn't need any marketing. He was setting PPV records with his very first fight. It's not the same with Fedor, who is still in the big scheme of things, a virtual unknown. Now with Werdum just putting a spanking on him, he is even less relevant. The casuals see him as the fat, out of shape guy who got beat by that guy that got cut from the UFC.