It has to do with the relative intensity of the event. While the marathon and the 5km both need basic aerobic training (around AeT), for the 5km this stuff is the non specific support training. For the marathon it is the event specific training since the marathon run is competed at the aerobic threshold. Thats right! The men running 26 x 4:45/mile are running right at their aerobic threshold (maybe a wee tiny bit above with lactate= 2.5mMol/L) just the same as the guy running a 3 hour marathon is also operating at his aerobic threshold. Its just that the 2:03 marathon can produce a lot more ATP at his AeT than can the 3 hour guy.
Now, XC skiing is different because the intensity swings are vastly greater than running especially on flat road courses where pace changes are small. In XC skiing the skier can exceed the maxVO2 intensity on some shorter climbs and accrue a large oxygen debit because that debit can be repaid as they glide down the other side of the hill allowing for recovery. This makes the intensity demands of even long races like the 50km very much higher than a comparable running distance. So the metabolic demands of a skier (Skimo too) are more heavily skewed to high intensity. This means that the basic aerobic capacity (amount of power at AeT) plays a supporting role and is not event specific training. Important none the less. But no event specific even for the 50k skier. This is why SXC skiers can do well in sprints and 50km races and we never see 5km runners winning marathons.
Hope this helps.