Security ... think about it!
The most common form of combat has been the ambush. The Afghans have used this tactic for thousands of years. The availability of modern explosives has made the roadside bomb another powerful weapon for their ambushes. The prime targets for ambushes are targets that can provide the least resistance. In other words, supply convoys. Foreign troops have to devote enormous resources to protect these convoys. This has generally worked, but it means that about half the combat forces are occupied with this. Still, the effort does a lot of damage to the Taliban, for the enemy keeps trying to ambush the convoys, and American (and NATO) countermeasures (helicopter or UAV scouts, combat troops with the trucks and smart bombs always on call) have made such ambush operations higher risk than the Afghans are used to. The Russians, in the 1980s, were much easier targets. But the Russians didn't have the UAVs, and most of their troops were poorly trained conscripts.