Legal drugs are usually pushed by huge corporations with big budgets. They have heavy marketing and a lot of advertising that manipulates public opinion. All of this advertising of course portrays the "drug" in a positive way. You don't ever hear the word "drug" used in alcohol or pharmaceutical commercials.
Also, whenever a study comes out saying anything good about alcohol the media picks it up since that's what people like to hear. I'm not sure I've ever seen the news feature a study on the negative effects of alcohol. Another reason the media may focus on positive stories is that big alcohol is such a huge customer.
Any advertisement mentioning illegal drugs portrays them in a negative way since they are put out by groups that are against the drug since you can't promote a product that's illegal. So in conclusion, it's because alcohol is legal and the loads of advertising that give people such a positive view of alcohol.
Two things don't need to be equally strong or powerful to each get the job done.
> MCDA modelling showed that heroin, crack cocaine, and metamfetamine were the most harmful drugs to individuals (part scores 34, 37, and 32, respectively), whereas alcohol, heroin, and crack cocaine were the most harmful to others (46, 21, and 17, respectively). Overall, alcohol was the most harmful drug (overall harm score 72), with heroin (55) and crack cocaine (54) in second and third places.
https://www.thelancet.com/journals/lancet/article/PIIS0140-6...
Of the myriad drugs I've taken, alcohol is the least deserving of ready availability IMO. I suspect that if it were invented post 1950, and not so ancient and ingrained in our culture, there's no way we would let people buy the stuff.