The answer for that question is incredibly simple in my opinion, however there might be many different correct replies you can make.

One of them is that gangs and criminal groups do want illegal drugs because that can make them very rich, the profits from drug traffic and drug dealing are enormous.

Another one is medical industry. They do not want to see weed legalized, because it can cure or help in thousands of different illnesses. The sales of their pills would have gone down, after all why would anyone take a pill for a headache if you can simply smoke a little bit of weed, to get  well?