Same reason people are scared of things like sharks.
Media, and common misconceptions.

Most people who are scared of my ball pythons seem to believe they'll strike at them, or strangle them.

-After 5 minutes of looking at me handle them they think differently.
-After touching them (sorta petting, like they would a cat) they realize they're not cold (amazed how many people think they're going to be freezing cold) or slimy, but smooth and room-temp.
-After more than one visit, most are even willing to try and hold one, normally a sub-adult.
-After a couple tries, I let them hold one of my friendly adult males or females.

Now all my friends tell others what they have learned, and I often have friends of friends wanting to visit for a look.

Once misconceptions are gone, people fall in love, or at least lose their fear.
I now have a bunch of people on any-time call updates for when my females lay a clutch, they find the eggs fascinating!

Another odd fact (personal experience only): males seems to be less afraid of them to begin with...However, when a male is afraid, he takes far longer than any of my girl-friends to man-up and hold one.