Simply addressing the title of the thread as opposed to the OP, why conservatives don't like marijuana:
It runs against the values instilled in formative years, such as high school. It's wrong. It's bad. It's illegal. You will go to jail for it. You will get a criminal record. You will never get a real job. You will ruin your life. Don't do it. Fast forward 30 to 40 years and these values have most likely changed little, consistent with a social conservative. (If you still do not qualify me as conservative, find any period in this text and count the number of spaces to the next character.) So I believe our views are slow to change with new information.
Had I known the effect on farts, then maybe it would have been a different story.