Over the past three or four decades, the illicit drug market has:

Hey there!
It's increased. By a lot.
Recently, and in the future, it's expected for the illegal drug market to grow even more. More people are getting hooked into the horrid and seeming temptations of drugs, and it ruins their lives.
Therefore, your answer is that over the past few decades, the illicit drug market had really skyrocketed, and it's expected to do even more. It's also become widespread. Not only is it in few big locations, it could be everywhere, and that's why it's harder to catch- it's expansion is hurting people.
Hope this helps!