Part of the problem is that the country has a short memory. Drugs of all sorts used to be legal in the U.S. The problems were so bad that these “recreational” drugs were made illegal. Marijuana is often compared to alcohol. One major effect that occurs with both is to decrease your inhibitions. A major difference is that when the alcohol is gone, so is this effect. When the THC is gone, some of that effect remains and it is cumulative. It especially affects the young, below 26 years old. This is because the prefrontal cortex is still in development. This is the part of the brain that helps you keep from doing stupid or illegal things.
I just look at long term marijuana users and I have all the evidence I need to form conclusions about them and about marijuana.