Because the UFC heavyweight used to be a division for an average of 240lbs. Look at all the good fighters, from Kongo, to Silvia, to Couture, everyone was walking around with 240 or so.
Then Dana comes up with a random beast, who just got on UFC, and probably took about 55 cycles while on the WWE to achieve his size, while most who are in the MMA world are tested ever since they are amateur and have to go at "normally". It's not a matter of how skilled he is, it's all about him having 50lbs on everyone else in the division, and him not having to go through the process all the other UFC fighters had to go through.
UFC fighters, heck MMA fighters in general, are not gonna be able to use the amount of gear Brock did to achieve his body size and build. And most fighters can't really take on someone who is like that, but Fedor has proven he can take people no matter what their size is.
So that's why, even though I think some UFC fighters could take him down with a few more pounds, like Mir, Gonzaga, and heck, even Kongo if he puts on another 10lbs or so. Most of Brock's fights have been taken over by his brute strength. Given, he's a great wrestler, but even great wrestlers sometimes have a hard time when fighting other great wrestlers ou takedown defense artists. Brute strength makes everything that much easier. Like GSP x BJ. The weight difference played a role there as well, and even though GSP is a much better fighter IMO, he still mandhandled BJ through brute force as well.