The mast camera is actually two cameras with zoom of each camera fixed at two different values:

http://msl-scicorner.jpl.nasa.gov/Instruments/Mastcam/

James Cameron lobbied to make zoom lenes for the cameras so they could be set at the same zoom level to create stereo pairs for 3D images.

JPL had "problems designing the lens without using wet lubricants which would require battery-sapping heating"

Difficult trade-offs must have been made everywhere - who knows where that extra battery power went that the zoom lens gave up.