You're essentially asking why Obama, a fairly centrist liberal by many accounts, didn't end racism forever.
There's a very good reason why black people are disproportionately arrested for drug-related crimes, especially Marijuana, despite having similar usage rates as others.
There's also the fact that members of the Nixon administration outright admitted this fact.
The reason Obama didn't do anything isn't that he's not that worked up over racism, it's just not as simple as waving your hand and saying everything would be great if it only for those racist laws. Drugs reduce inhibitions. People with reduced inhibitions do crazy things, often hurting other people. Families are often willing to support draconian measures so that their kids aren't endangered by some random stranger in some drug induced state of manic paranoia.
There's a very good reason why black people are disproportionately arrested for drug-related crimes, especially Marijuana, despite having similar usage rates as others.
There's also the fact that members of the Nixon administration outright admitted this fact.