Here's the deal with their thinking according to Meltzer: Brock Lesnar costs them a bazillion dollars a match, so the only chance they have at recouping their costs in the deal is to put him in the ring with the biggest star they have available at the time (or on the outside have him completely destroy a minor star like Big Show) and at the moment that would only be John Cena. They're not paying him all that money to make other guys into stars, because that's not their mindset. If Rock was around, or Batista, for example, that would be justifiable. Anyone under that level, to them, is just making Brock less special.
NOW, that being said, I think they're full of crap because the way that wrestling is supposed to work is that the bigger stars use their star power to make the smaller stars into bigger ones. But when you're booking for investors and using soap opera writers, obviously that mindset kind of gets lost in the shuffle.