You know, I didn't claim to understand the reasons behind the technical limitations, but the reactions of some of you are puzzling to me.
We can go on and on about familiarity considerations, and planning considerations, and instrumentation considerations, and scientific concerns, etc. It all sounds like excuses after a bit.
Let's reformulate the question: Why not keep the same enclosure, the same lens assembly, the same mechanical parts and simply swap the 2MP sensor for a 20MP sensor?
Further, let's say you are really not sure about that spanking new 20MP sensor, because it is new, you are not familiar with it, and it was not part of the plan drafted 10 years ago, why not add 1 extra camera with a 20MP sensor? If it blows up during the trip or during landing, too bad, it's lost. If it makes it down there safe and sound, you can collect a 10-minute HD sequence that you can stream back to earth over the next 2 years, during down-times, as a very low priority task.
Potential benefit: You offer the world the first and a truly spectacular HD film shot on Mars, in all its splendor, and you inspire a new generation of world explorers, who will ultimately drive man to take another small step, but a giant one for mankind.
Clearly, if that's the best we can do for something as trivial as shooting a video up there, I cannot fathom a man setting foot on Mars in my lifetime. That may be normal, but it saddens me somehow.