Rising Epidemic - Marijuana

Marijuana is the most frequently used illegal drug in the United States. Marijuana damages brain cells, distorts perception, increases the heart rate, and is associated with memory and learning problems.
http://healthcenterinternationalresearches.webs.com/brain-addiction-2b.jpg
Also, studies on marijuana show that it is a 'gateway' drug, or in other words, it leads people to try other more serious drugs. Nevertheless, evaluating whether it is a gateway drug isn't easy. I feel more studies needs to be done on this important topic. With such negative effects, why is it still the most frequently used drug? More studies and results need to be done in order to show the country this is a rising problem that should be taken more seriously.
http://www.heritage.org/research/reports/1981/05/the-marijuana-epidemic