As a political science major, I am quite familiar with this subject. The reasons for ending drug prohibition are numerous. I will not list all of the reasons here, but I will list a few major reasons:
A decrease in crime. Ending drug prohibition would decrease crime in a couple of ways. It would eliminate the main source of income for most gangs. Without this income, gangs will not have the resources to purchase weapons. It will lower the costs of producing drugs thereby lowering the price to obtain drugs. If drugs are less expensive, drug users are less motivated to commit crimes in order to obtain drugs.
The economic cost of drug prohibition. The War on Drugs has not only led to an explosion in the prison population, but has also led to much money being wasted on ineffectual domestic and foreign drug prohibition enforcement. Ending prohibition would also yield a net increase of aprroximately $15 billion (contingent on eliminating enforcement efforts and marijuana being taxed the same rate as alcohol and tobacco) for the federal government.
There are also philisophical reasons to end drug prohibition. The belief that what one does in private should not be regulated by the government. That a consenting adult should be able to do whatever they want with their bodies, including the recreational use of drugs, as long as they do not harm others.
I could list more reasons, but I believe these reasons suffice.