I think it's because in day to day life, there aren't many obvious things that allow people to talk to strangers. I think people would talk to each other in the street all the time if they were more open, and inviting. Many small communities are like this. But in those that aren't, I think the opportunity to chat up a stranger presents itself when you're a tourist/tourer. The opener is obvious, and easy, so they do it without hesitation. It's rare enough that people are curious and take advantage. Also, most people traveling by this method are kind, salt of the earth people. Who wouldn't want to have a chat?

It's one of my favourite things about touring by bike. Always a positive experience.