Why did the Russian invasion of Afghanistan fail?

The Russian invasion of Afghanistan failed for many of the
same reasons  that the US war in Vietnam failed.  In both cases, the superpower
underestimated the difficulty of fighting against a guerrilla insurgency in difficult
terrain when that insurgency was highly motivated to
fight.

In Afghanistan, the resistance to the invasion was
fierce and highly motivated.  The resistance was motivated at least in part by religion,
which tends to give people a very strong incentive to fight hard and not give up.  The
Russians did not understand that the Afghans would be so strongly motivated and so they
did not understand how hard the war would be.

The Russians
also did not understand how unmotivated its own troops
would be.  This was partly because many of the Russian troops were Muslims from the
Central Asian part of what was then the Soviet Union.  These troops were not strongly
motivated to fight against fellow Muslims.

So the invasion
failed because it is hard to fight in Afghanistan, because the Afghan resistance was so
motivated, and because the Russian troops were not motivated.