Several reasons but one is a hangover, if you will, from the Russian campaign. The conscripts are young and in a country as poor as Afghanistan, are "lifers".
They end up with a strong national identity and see the drug trade as a threat rather than an opportunity. They also have the example of Pakistan right next door. The Pakistani army is nearly a entity in its own right whose primary purpose, or one of them, is to protect the national identity when necessary from internal political upheaval.