Why are things like prostitution and marijuana illegal?

These things are illegal because elected legislators, such as state senators and representatives, have enacted them into criminal law. These kind of laws can be changed or repealed. People in many states are trying to have the marijuana laws changed, such as to legalize marijuana use. However, the laws might still collide with federal laws. If people want to change these kinds of laws, it is important to register and vote and get the kind of representatives that you want.

Marijuana and prostitution are legal in some states. Traditionally, each state has been allowed to decide which "vices" it will allow and which it will forbid on the grounds that they pose too great a risk to some or all of society. The vices that are allowed are usually taxed very highly as a means to discourage as many members of the public as possible from participating in them. Whether an act is a vice or not changes over time, with society's changes in perception of harm, and with evidence provided by research. Something like prostitution may be enjoyable to some but many of the people who serve as prostitutes are forced into it and often at a very young age. It's neither safe nor risk-free to them and many people now consider it a sign of human trafficing (slavery) which is always illegal. Some vices are evidence of other, more serious crimes that are harder to stop or even to identify the participants. Arresting someone for the illegal vice can lead to discoveries about the more serious crimes.