Runners who excel at 5k road races or 10k on the track are not necessarily the same ones who excel at cross country running. Andre Kaokane at Cross Country Running Shoes 101 says that the two disciplines require different techniques and styles.
The ground in cross country is softer and more slippery, so you need to keep a shorter stride length. If you try to use the same style of foot strike as in road running (with the heel striking the ground first, well in front of the body), then you are quite likely to skid over the softer terrain. Using good cross country shoes, with a larger tread pattern than road shoes, will help to provide the necessary traction.
Similarly, if you allow your rear leg to be placed too far back, like in track running, you will be left with a weaker push-off. Shorter foot strikes require a faster leg turnover, which is more difficult because the softer terrain provides a less elastic rebound.
Running on the track, much energy is stored in the leg and ankle joints by the compression of tendons and ligaments. However, in cross country, that energy is largely lost to the compression of the soft ground underneath. Since less energy is stored in the joints, the runner must compensate by bending the legs more at the knees and ankles, and then straightening them again. The runners must also expend more energy picking up the thigh muscles, which creates more work for the pelvic muscles and places additional strain on the abdominal muscles in order to hold the torso upright.
An efficient track runner can make the job easier by using the rebound forces of the hard surface to coast along while maintaining an even balance because of the level surface of the track. The cross country runner, on the other hand, has to rely on the brute force of muscles. The soft surface provides less rebound, and the uneven terrain requires a constant adjustment of the muscles in order to maintain balance.