Nobody knows.
Based on my reading of what experts say, and my own observation, different people have different susceptibilities to alcoholism from the beginning. But not everyone who is susceptible is as easily susceptible. I know many people who report an extreme abnormal reaction to alcohol the first time they drank it. So that piece of it seems to be genetic.
For those who don't react in an extreme fashion from the beginning, it seems to take greater exposure to it before the addiction kicks in. For those people, obviously if they seldom had occasion to drink much, they wouldn't become addicted. So there might be cultural or environmental factors that lead people to drink enough that they do get addicted. (I'm one of those who always sort of had a tendency to overdo it, but it wasn't until I drank a lot for a long time that I think I truly became an alcoholic). In those instances, it can be very sneaky--you don't realize you're addicted until it's too late.
I think every alcoholic has some combination of genetics plus environment that adds up to alcoholism. It's just that the combination is different for different people.