Its more than just choosing the camera 10 years ago, its having 10 years of testing of that particular camera under their belt and knowing every in and out of it. 10 years is plenty of time for any quirky behavior to emerge and be dealt with.

Several articles have mentioned bandwidth limitations of about 250 megabits per day, which also has to be used to send instrument data.

But I haven't seen an explanation why bandwidth is so low -- is it lack of spectrum? Interference issues?

It's not like they have extra batteries and they just add a power strip for the better camera. The change you suggest would involve re-engineering, which would increase your lamented cost of this mission.

Maybe Instagramm built the camera?

But seriously, I guess for the same reason Couriosity runs on old PowerPC processors. They are just well tested and less likely to fail. If you want a high res image, you can always take 10 images and glue them together, result is the same.

This topic was also discussed here (regarding some previous missions): http://news.ycombinator.com/item?id=4356878

They use a black and white CMOS to accurately measure the actual light hitting the lens (which may include non visible spectrum. To get a colour image they need to use filters and take multiple photos, where as consumer cameras have a fixed grid (Bayer pattern) of one filter per each pixel. That probably won't give you such an accurate picture. Also when they're using filters they're not interested in reproducing true colour but rather highlighting a difference in materials.

The bandwidth constraints no doubt the reason they didn't change the sensor. Besides, when they combine three different images taken with filters to create one colour image it will essentially be 6MP anyway.

More like A) We aren't getting the pictures from the best cameras yet and B) We didn't send the latest, greatest best cameras because we didn't want them to fail after 10 minutes use.

We send up camera's tested and hardened against temperature changes, dust, vibration, and radiation. Also cameras that require low power.

Furthermore, there is limited bandwidth. So we probably want to send back mediocre pictures most of the time and if we see something we really want in high quality we take a bunch of pictures of it and put them together.

As a final note, the quality of the sensor and lens probably matters a lot more than the number of mega pixels.