Reasons I can think of include: the ground does not consist of a regular pattern of known length so image processing might be more difficult; looking out over the track allows more data points to be interpreted; an outward looking camera can fulfill more additional roles or be repurposed to compensate for other failed instruments more readily; a downward facing camera is more likely to gather images within the rover's own shadow; other outward facing cameras could use the same algorithm should the distance measuring camera fail.