This is a good question and as you can imagine there is no one answer. Also the answers will be generalization. In light of this, let me give you a number of reasons.
First, life circumstance is an important factor. Some people go through a hard time in life and when this happens, drugs can be a way of escaping. For this reason some may resort to drugs.
Second, accessibility is another factor. Those who grew up around the easy accessibility of drugs will probably take drugs more than those who grew up in a place where drugs are hard to come by. For example, if a person grew up in a culture where their friends are taking drugs, then it is quite possible a person can experiment.
Third, some people find that taking drugs is a way to rebel, especially young people.
In the end, the story of all people is different.