Farming requires patience and round the year hard work. People in Pakistan have harvested these lands for thousands of years. It's in our blood and part of the culture now. On the other hand, Afghans are different. You have to look at it from their perspective. There is lot of poverty and no strong central government. A common Afghan has to deal with corrupt officials, local warlord men and insurgents, all at the same time. He is in desperate need of cash too. Growing poppy is best bet for him. It brings lot of cash in short time. Government offocials, warlord and insurgents, all take their cut on his income so they also promote poppy cultivation which generates cash quicker.
Drug abuse has been a major problem in Afghanistan and some adjacent parts of Pakistan.