The question of why weed or ecstasy is not legal is a tough question to answer without speculation of bias one way or another. For many people, the country's experience with cigarettes and alcohol present an example of legalization leading to government revenue, etc. which is one of the leading arguments behind legalization of marijuana and other drugs.
The issue with legalizing certain drugs (like marijuana) stems from the reasons behind marijuana use. The truth is that drugs such as marijuana, which can easily be grown by the consumer, will not profit the government in any way. The ability for street dealers to circumvent any imposed tax far outweighs the negative effects of having drugs available on the open market. Once they are legalized, the individual states must then begin to impose new laws for operating motor vehicles under the influence of said drugs, as well as appropriate levels that still allow a person to operate vehicles without sufficiently inhibiting their ability to safely navigate the street.
Furthermore, one must look at why people consume marijuana and ecstasy as much as they do. In my experience, the use of these drugs is fueled mainly by the fact that they are illegal. While this may seem like an argument for legalization, it's actually a reminder that, if these drugs are legalized, the population will then look for the next illegal thing to consume. Once marijuana loses its value of intrigue to a person, they'll soon look to alternative drugs, whether that be cocaine, heroin, or other drugs on the market.
All one has to do is look at the country's image towards alcohol and cigarettes. Now that both are legal, people no longer see them as fashionable outside of college parties (where minors are still attracted to alcohol).
If certain drugs, such as marijauna and ecstasy are legalized, the country is only opening the door for its youth to pursue harder, more dangerous drugs. As long as marijuana keeps its intrigue to people, they'll have no reason to try other drugs, and will therefore not run the risk of seriously hurting themselves.