One simple reason is addiction to drugs.
Street children feel that using drugs will solve their problems which obviously is not the case. At the most, drugs take away their hunger pangs in the street-life and also it takes them away from the harsh reality for a short period of time. Then, bang! back to the same situation that they left behind before getting intoxicated.
Drug abuse by street children of Nepal includes alcohol, cough medicine, heroin, marijuana, and dendrite. Dendrite(industrial glue) and marijuana are the most used amongst those.