#2 To prevent discontent in the locker room
WWE enjoyed a monopoly for many years, but they are not the only game in town anymore. And WWE has often been accused of bringing in big names from the mainstream world and pushing them ahead of full-time members of the active roster. And whenever this happens, there's always a sense of discontent in the backstage area.
One has to assume that people wouldn't be happy with Cain Velasquez becoming WWE Champion in his first match. We all hear stories about how angry the locker room was back when Goldberg was pushed ahead of experienced stars in WCW. The same thing could have happened in this case too.
A happy locker room always makes for a better product as we are well aware. Brock Lesnar may not be a part of the full-time roster, but he's been around long enough, to have earned his position in the company.