In case you haven't seen how this thing lands, watch the link below. It's really so amazing how this is entirely automated and executed so flawlessly.
is a paper written by the engineers who designed the system, explaining these decisions. I suspect that the reason is that - in Mars's thin atmosphere - 500kph isn't as much pressure as it would be on Earth. (The constraint appears to be that the heat shield needs to fall away.)
I found the way MSL detects landing to be clever. Instead of a switch or sensor, it records the throttle setting required to descend at a constant speed. When that setting drops (for a sufficient time), the rover must have touched down.
http://www.universetoday.com/7024/the-mars-landing-approach-...
For instance:
“We call it the Supersonic Transition Problem,” said Manning. “Unique to Mars, there is a velocity-altitude gap below Mach 5. The gap is between the delivery capability of large entry systems at Mars and the capability of super-and sub-sonic decelerator technologies to get below the speed of sound.”
The article is in the context of landing humans on Mars, but the principles are the same.
I'm curious - wouldn't a large, variable geometry lifting body do the trick? Land like a shuttle would, but with a bigger wing, and deploy more wing and lift devices as you get lower.
Besides the other commenter who mentioned the size of the lift surface, think of the mass requirements, the variable atmosphere density mentioned in the linked article, the need for an autonomous lander (because of light time), and the small amount of time you have to land.