Because Mars doesn't have a thick enough atmosphere to parachute-land a big spacecraft. The atmosphere is thin not only in density but in altitude.
It wasn't until about 15 years ago that it was realized that we had no clue how to land a large craft on Mars. "Plainly put, with our current capabilities, a large, heavy vehicle, streaking through Mars’ thin, volatile atmosphere only has about ninety seconds to slow from Mach 5 to under Mach 1, change and re-orient itself from being a spacecraft to a lander, deploy parachutes to slow down further, then use thrusters to translate to the landing site and finally, gently touch down." - JPL.[1]
The approaches used for the small landers don't scale. Giant supersonic parachutes a hundred meters across don't look workable. There are some exotic parachute/heat shield concepts such as "hypercones" which have never been tried. Deceleration outside the atmosphere with rockets takes huge amounts of fuel, but should work.
The refueling in orbit plan makes sense. (Sending the fuel tank up first would be simpler, if less impressive visually. Build up fuel storage in orbit with multiple launches. This was von Braun's original lunar plan.)
It wasn't until about 15 years ago that it was realized that we had no clue how to land a large craft on Mars. "Plainly put, with our current capabilities, a large, heavy vehicle, streaking through Mars’ thin, volatile atmosphere only has about ninety seconds to slow from Mach 5 to under Mach 1, change and re-orient itself from being a spacecraft to a lander, deploy parachutes to slow down further, then use thrusters to translate to the landing site and finally, gently touch down." - JPL.[1]
The approaches used for the small landers don't scale. Giant supersonic parachutes a hundred meters across don't look workable. There are some exotic parachute/heat shield concepts such as "hypercones" which have never been tried. Deceleration outside the atmosphere with rockets takes huge amounts of fuel, but should work.
The refueling in orbit plan makes sense. (Sending the fuel tank up first would be simpler, if less impressive visually. Build up fuel storage in orbit with multiple launches. This was von Braun's original lunar plan.)
[1] http://www.universetoday.com/7024/the-mars-landing-approach-...