What percentage of Americans even know that alcohol is a drug? As a Wisconsin native, I've seen first hand the toxic culture surrounding drinking in America, including having a relative die of alcoholism. Still, we don't seem to look at it like other drugs, even though it ruins lives just as effectively as something like heroin.
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.