JDS and Cain are also extremely easy to relate to for the a lot of people. Cain is young, Mexican (getting that Latino demographic really well), and he is the force that stopped Brock Lesnar and made him seem like a 4th grader in high school. JDS has that "nice guy" image that GSP has too, except not as exposed as GSP. He is also young since comparatively, 26 is still young in MMA. I'm bringing up their ages because these guys will be in the UFC for a while, so why not start the FOX deal with guys who the viewers will be following for the next decade. This is better than starting with established guys who may or may not be in the UFC in the next couple years.
Plus, as everyone has said, both guys like to actually fight and throw down rather than fight to win (like GSP). Zuffa will be hyping this fight up showing Cain stopping Brock and the KOTN he had on Big Nog (which after Nog's win last weekend is pretty huge), and for JDS showing his KO of Gonzaga, the striking clinic he gave Roy Nelson, and the destruction of Carwin. IMO the easiest way to get someone into MMA is to show excellent striking and "badass" status.