Drug dealing in a society where drugs are criminalised is a way to make a lot of money. As a result, young people among the poor find selling drugs an easy way to earn their living and escape the more demanding path of working in an honest job.
This causes a bad effect on the societies in poor neighbourhoods, who turn to drug dealing instead of advancing their rank in the honest world. Making drugs legal will kill this easy way out, and force the young people of the poor to struggle for a honest living and gradually advance their personal and collective financial status.