4 ideas
A mechanized force that must supply itself along a few mountain highways turns its own logistics into a predictable target, because terrain fixes where convoys must pass and where ambushes can be laid. The guerrilla does not need to beat the army in battle. It only needs to keep making the road costly, forcing the stronger side to spend its combat power guarding convoys instead of hunting fighters.
Irregular fighters who depend on captured supplies are tempted to stop and strip a destroyed convoy instead of withdrawing at once. That pause gives the enemy time to bring in helicopters and relief columns, turning a successful ambush into a costly one. The need to plunder conflicts directly with the need to break contact fast.
Mujahideen groups were rooted in their own valleys and relied on local villages for food, recruits and intelligence. This made them resilient and hard to root out, but it also made them reluctant to fight far from home or to combine with rival factions. Read this way, their local strength and their inability to concentrate for decisive operations have the same cause.
Soviet forces answered the insurgents' dependence on villages by bombing and emptying rural areas, pushing millions into refugee camps. This raised the insurgents' logistical burden, since fighters had to carry in supplies that locals once provided. It also deepened hatred of the occupier and fed recruitment from the camps, so the counterinsurgency both weakened and strengthened the resistance.