You are comparing the very first initial images from Curiosity with the final processed images from Viking. Also, the haz-cams on Curiosity are there only to help Curiosity see where it is going. Whatever design trade-offs they make, they need to make the priority be "don't break the 2.5 billion dollar rover that is supposed to last for over a decade."

If you use a low-res camera to take 1000 images of the same thing, you can use software to make a high-res image from those.