The main reason for the MSL driving slower than physically possible is hazard avoidance (and also navigation/visual odometry):
Source: Introduction to mobility for MSL Heverly, Matthew C. 10-Sep-2012
Hazard avoidance and navigation
Drive rates (m/hour):
Absolute top speed: 151.2 (On rigid terrain with zero slip)
Blind drive: 139.5 (top speed for straight driving
with pause between steps)
with slip checks: 116.0 (Averaged over a significant number
of slip checks)
Visual Odometry (VO): 64.5 (At every step, look around,
see how far the rover has driven)
navigation (Autonav): 40.1
with slip checks: 38.5
Autonav with VO: 30.0 (Assumes VO driving rate with
additional time for map imaging
at the appropriate spacing)
What is Visual Odometry? Taking stereo images "before" and "after" the step to track true motion, calculate slip and embedding. Accuracy of VO is 3% of the step distance, so you wouldn't want steps to be too long.
Why is it capable of moving slowly?
Actuator design constraints
The mission design logic (see http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/38317/1/03-2974.pdf) is as follows:
- Interesting landing site => Technically feasible landing error ellipse => Traverse length
- Number of samples
- Nominal mission duration (one martian year)
Hence, $Nominal\ Speed = Traverse\ Length/(Mission\ Duration - Number\ of\ Samples * Time\ per\ Sample)$.