why do you think that is?
This is just my theory...
Town folk have a greater sense of community because they live within a small population. Hence, they're more likely to help each other out and will therefore, pick you up.
City folk tend to be less communal because of the greater population density of a city. I think people living in cities are more likely to experience a diffusion of responsibility... I speculate that they are less likely to pick you up for reasons such as: they think someone else will do it, they're too busy (their lifestyle is more fast paced), they don't like people who travel like we do, or they just don't care...
And of course, both city and town folk won't pick you up because they're scared and/or don't like people who hitchhike.
There are many more reasons as to why someone won't pick you up, but you get the idea...