Marijuana has long been the gateway drug to more hardcore drugs. You will not find an addict on the street that did not start with marijuana. Due to its cost and ease of use it is primarily the first drug anyone tries. Once you can't get high enough you start to look for other ways.
Marijuana has a reputation as a gateway drug because it is considered the "lowest" level of illegal drugs. In reality, most addicts use illicit drugs to "self medicate" for underlying psycological issues. If marijuana was not available, a true addict would just begin with another type of drug, be it prescription pills or other illicit drugs.