It scares the hell out of pharmaceutical companies and stakeholders of major breweries. How are people going to die in car accidents or become belligerent assholes if they have the option of the one drug that doesn't kill and is non-addictive? Lobbyists have asked the very question you have, and the more they fear MJ's public perception taking a positive turn, the more they will exercise their financial leverage.Below is a Youtube video of an older man that has high levels of pain on a daily basis.
MJ is the only thing that helps alleviate his pain and allows him to live a somewhat normal life. This has to scare the HELL out of the pharmaceutical companies. If people can self medicate legally, how are they going to sell there oxycontins, zanex and other stuff?
The "gateway drug" argument IMO is horseshit. If alcohol were illegal, it would be used in the same context that increases likelihood of "hard drug consumption" as marijuana does now. The counter-culture of pot-smokers in illegal states are naturally predisposed to reckless activity, given the nature of their illegal activity (and/or business)...hence their social circles are more prone to hardcore drug experimentation.
There is no slippery slope with this substance in the broad scope of "drugs." Most of America has "grown up" and gotten past the bubble of old-school thought that "drugs are bad" ...therefore "pot is bad" thinking. We as a culture have drawn the line in the sand of what type of substances will never be available on the market.
To be quite honest, the issue is about as controversial as gay marriage today; I know you all are smart enough to know the latter issue is not even debatable.