As previously reported, WWE is still looking to book Brock Lesnar vs. The Rock for WWE SummerSlam. Word is that there was once discussion about doing the match for WrestleMania 29, but that was when Lesnar's deal was set to expire. Now that Lesnar has signed on for another two years, the push is to have the match at SummerSlam.
One thing that got the match changed from WrestleMania 29 was that WWE wanted The Rock in the WWE Title picture and Lesnar's schedule was seen as an obstacle to booking a WWE Title feud between the two. Another reason that WWE is looking to have that match at Summerslam is because both have mainstream appeal, and that with them in a featured spot, the local Hollywood community could get more involved with the event. An example would be having celebrities appearing at the WWE AXXESS event.