In a recent press conference (think it was this one: http://www.ustream.tv/recorded/18431005 http://www.ustream.tv/recorded/18431005) one of the MSL engineering

In a recent press conference (think it was this one: http://www.ustream.tv/recorded/18431005 http://www.ustream.tv/recorded/18431005) one of the MSL engineering managers was asked this question. He said it's because of both the weight of MSL and the landing accuracy requirements. They considered a boatload of configurations before deciding on this one.