Taxes are just a wealth transfer item that can be used as bartering chip in the collective negotiations we call politics.
One group wants something (doesn't matter what it is) but let's say cannabis legalization. Another group might have no reason to care about that, but since the status quo is illegal, might as well extract some value from the first group. Never give up something for nothing. As such, cannabis taxes are included in all of the legalization bills.
This is the important part: the reasoning comes afterwards. Cannabis will make people unproductive, it will increase car accidents, the whole place will smell like pot, etc. Those are all reasons. There's data in some form to support all of them, but none of them are the real reason, which is that it's just good business to ask for something in return, and take as much as you can get away with.