I've not found the papers that discuss the design of a direct descent trajectory to the Martian surface, but I can make an informed guess about why it was done as it was.
As sts60 pointed out, the interplanetary trajectory was designed simply to get MSL to Mars using the least amount of fuel. This required launching when Earth and Mars were in the right positions, something that happens for only a few days at a time every 2+ years. It also required launching at a specific time of day when KSC passed through the plane of the desired Earth parking orbit. As with Apollo, MSL first entered a low Earth parking orbit, coasted, and then fired its upper stage a second time to inject it into its trajectory to Mars. And in both missions the launch azimuth (direction) and coast period were adjusted to account for the exact launch date and time.
The trajectory designers could have aimed MSL anywhere on or near the Martian disc as seen by the approaching spacecraft. Because it was headed for Gale Crater on the equator, it obviously had to target some point along the equator. But where? Had it hit the center of the disc, it would have entered the atmosphere at a very steep angle and been quickly destroyed by the extreme deceleration and heating. It had to shoot for the edge of the disc, just Apollo did when returning from the moon. During cruise MSL's speed was adjusted very slightly so it would arrive at this point above Mars just as Gale Crater was passing under it.
But which edge? If you shoot for the 'left' or western edge (with north 'up'), your velocity relative to the Martian atmosphere (which rotates with the planet) would be your own heliocentric velocity plus that of Mars own west-to-east rotation. If you shoot for the 'right' or eastern edge, then Mars' rotation speed subtracts from your own velocity and your velocity relative to the atmosphere is lower. That makes the entry much less stressful, so that's the one you choose.
So you shoot for a target point somewhere in the Martian atmosphere that is low enough to ensure capture but not so low that you descend too steeply and burn up. It was just like an Apollo re-entry from the moon except that Mars has only 1/3 Earth's gravity and 1% of Earth's surface atmospheric pressure. The Apollo entry angle was 6.5 degrees; anybody happen to know the figure for the MSL entry?
This created one unavoidable problem: as seen from Earth, MSL went around the right edge of Mars shortly after entry, making it impossible for MSL to communicate directly to earth all the way to the surface. Fortunately, JPL arranged for the active Mars orbiting spacecraft to be in the vicinity at the time so they could relay real-time communications to Earth. This worked remarkably well; without them we would not have known about the successful landing for another 12+ hours when the landing site came around the other edge of Mars and in view of Earth again.
