I think there are many reasons why people are scared of snakes.
Instinctual: Lets face it, many animals are leery of snakes so why not humans?

Upbringing: If you're taught snakes are dangerous, deadly, evil, the devil (yes I've seen people literally scream and cry that a garter snake in their yard was the actual devil himself coming to get them), etc. then most people will continue that thought process throughout their lives.

Lack of knowledge about snakes: This is the biggest one in my opinion. People are scared of what they know nothing about. 99% of the population couldn't tell you jack about snakes, other than what they've learned from Hollywood and urban legends. So why wouldn't they ask "does it bite" or "is it poisonous" and "are they slimy"?

Bad experience with a snake or snakes: If someone gets mauled by a German Shepard chances are that person will be wary of the next German Shepard they come across.
My grandmother is scared to death of snakes. This stems from a bad childhood experience in which she was playing outside and fell down onto a large snake that she did not see and it hissed and bit her and then squirmed all over her.
If you followed the 'Indiana Jones' movies you learn in the 3rd one how he became scared of snakes, which stemmed from a bad experience.

Finally you just have phobias. There is no reason what-so-ever for them to be scared but they just are. Just like some people are scared of large open spaces, or closed in tight spaces, the dark, the sun, etc.

I understand it and don't blame them, heck I'm not too fond of spiders. But what I can't stand are the people who then go out of their way kill any and every snake they see.

Oh and there will always be those who don't listen to their parents or are curious and will try to learn about something instead of fearing it, or who were raised differently. So that's where we come in.