Each of Curiosity's wheels have a drive motor, but only the four outer wheels are steerable. The suspension system is based on rocker-bogie system on the Pathfinder rover.
The term "rocker" comes from the design of the differential, which keeps the rover body balanced, enabling it to "rock" up or down depending on the various positions of the multiple wheels.
Looking at the design of the rover, from the picture in the question, the location of the center wheels is dictated by the width of the payload chassis; more particularly, the "axle" beneath the payload chassis. These wheels cannot be any closer to the body of the rover.
The outer wheels could have been placed closer to the body of the chassis by reducing the angle of their support beams. This would have reduced the stability of the rover. It's the same principle as the stability of wide gauge railroads compared to narrow gauge railroads. Additionally, placing the outer wheels closer to the chassis would have increased the disparity between them and the center wheels.
If the outer wheels were placed in line with the center wheels the angle of their support members would have been greater than what it is. This would have increased their lengths slightly, which would also have increased their masses and thus the overall mass of the rover.