How the Curiosity rover will land on Mars from a sky crane

"Airbags were never an option for MSL, which is the size of a car and weighs 1,950 pounds, almost five times as much as Spirit and Opportunity (also known as the Mars Exploration Rovers). Airbags cannot be scaled up enough to support MSL and still fit inside the launcher. And previous airbag-equipped landers made unsteered, ballistic entries, resulting in a relatively large landing zone "footprint," or error ellipse, limiting the number of possible landing sites."