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.