Hitch-hiking used to be very common in the US up until about the mid '70s or so. During WWII it was actually encouraged by the government. Then, in the mid '70s, things started to go kinda crazy, and the perception developed (rightly or wrongly) that picking up hitchhikers was a very dangerous thing to do. It was around that time that we started to hear a lot of news about stalkers and mass murderers, and it seemed like the US had become a more dangerous place.

I think that a part of it is that one must admit that a very large number of people one sees hitchhiking today do seem vaguely dangerous in their personal appearance. Back in the earlier days, you used to see lots of young men hitchhiking. They would all be well groomed and clean shaven, and wearing clothes that didn't look too bad. Many would be college students, or one would sometimes even see a young soldier hitchhiking home on leave, in uniform; such would be offered a ride instantly. You just don't see that now, and because what few hitchhikers one does see appear to be so threatening, people simply are just out of the habit of offering hitchhikers rides.