The attraction is they would sell an addictive substance. This is why companies like Altria (nicotine) and Coca Cola (caffeine) have done so well for investors over the decades. If deregulation occurred, growth could go viral as more addicts are created.
The repulsion is that when a sector draws lots of enthusiastic public attention and declarations that it must be The Next Big Thing, there is a natural incentive for people to throw together some incorporation papers, write a 10k with stock photos of industrial production going on, collect millions of dollars from individual investors, pay themselves millions of dollars in salaries and options, and then fold alongside everyone else when the hype dies. The dot com, gold mining, and clean energy industries are examples of this dynamic. Penny stocks and busted dreams galore. Second, I'm not sure what the first-mover advantage would be for grass production. Upon full legalization, what moat would these little companies have to prevent Altria, Monsanto, or any multi-billion dollar pharma company from taking market share and out-innovating the established players? What would stop regular farmers from beating them by selling hay bales of dope at your local farmer's market? After all, a quarter bag is easier to produce than a half pint of whiskey (addictive substance = alcohol) and look how fragmented and low-margin that industry is.
As Retire-Canada points out, we're all better off in the indexes than we are making bold predictions and stock picking. Beware the "wisdom" of crowds and resist any "theme" the financial media hypes. Also watch for fly-by-night public companies.