This is essentially down to the difficulties of capturing and returning data from Mars orbit rather than Earth.
These images from MRO are taken by the Mars Color Imager (MARCI), a very wide-angled camera designed to make a (near) global map of the planet at low resolution (1-10 km per pixel from MRO's ~300 km orbit), primarily for tracking Martain weather. Compare this to MARCI's more famous co-passenger HiRise which is designed to produce very high 30-60 cm per pixel resolution images of much smaller areas.
From MARCI's (much less flashy) website:
Black areas in the movie are the result of data drops or high angle roll maneuvers by the spacecraft that limit the camera’s view of the planet. Equally-spaced blurry areas that run from south-to-north (bottom-to-top) result from the high off-nadir viewing geometry, a product of the spacecraft’s low-orbit...
Data drops are part and parcel of interplanetary missions partly due to the challenge of aiming at a distant Earth with the craft's antenna as well as signal drop off, atmospheric attenuation and interference.
The high angle rolls are often necessary for a variety of reasons:
- The above mentioned aiming of the high-gain antenna.
- Ensuring MRO's solar arrays are angled optimally to the Sun.
- Targeting surface features with the narrower-angled imagers on board, particularly HiRise:
Targeting requires rolling the spacecraft up to 30° off-nadir in either direction, waiting (several minutes) for the spacecraft to settle, commanding the instruments to observe, and then rolling back to nadir orientation.
These high-angle rolls may explain the smoothly curved edges of some of the missing data.
I believe the conspicuous horizontal lines through some of the blank regions are due to a combination of how MARCI captures it's images (essentially a slow rolling-shutter scan) and data drops.
More information here, particularly pages 5 and 10.