I actually just got done submitting a paper on this for my Ethics class. There's actually a few reasons why people are still so reluctant to legalize. For one thing, we don't clearly know the long term side effects. Studies have been done, but few things have been conclusively proven. However, as was recently published, some studies have shown negative long term effects, as Kai MacDonald and Katherine Pappas (2016) detail in Why Not Pot? A Review of the Brainbased Risks of Cannabis “…we introduce a mnemonic, DDUMB, to help remind us of the five brain-based harms associated with marijuana use: dependence, driving impairment, underachievement, mental illness, and bad to worse (i.e., marijuana serving as a “gateway” function for other more dangerous drugs)….”
Secondly, there is a similarity between smoking a joint and smoking a cigarette. We are trying very hard to stop people from smoking cigarettes, and to many people it makes no sense to legalize one while trying to stop the other.
Lastly, safety. It's easy to test for alcohol, but much harder to test for marijuana quickly when a police officer pulls over a driver. At the moment, we don't have the legal or testing infrastructure to quickly test and arrest a potential DUI driver. Driving while high is extraordinarily dangerous, just like driving while drunk.
Charlie Gillis (2016) mentions the danger of driving in Doobs of Hazard, paragraph 18:
"Trials performed in driving simulators have found that pot reduces drivers' ability to stay in their lanes and manage their speed… High drivers also tend to obsess on one task-say, their following distance from cars ahead-while ignoring equally important factors such as the positions and speeds of the vehicles around them."
Gillis also mentions that marijuana is much more difficult to test for than alcohol. At the moment, officers cannot field test for marijuana. If a person is suspect, then they need to be arrested and submitted to more rigorous tests, which takes a lot more time and money.
There are more reasons, but I'm running out of room to list many more At the moment, there are still too many issues with legalization to make it realistic at the federal level. While there would be a large profit to be made, until we solve these issues, it's just not possible. I hope I answered your question.