I spent a semester abroad in New Zealand and there were people there that left their doors unlocked. Similarly, in college a lot of my classmates left their doors unlocked even though the college administration repeatedly told us not to.

I think it's a density thing. In a small town, there's nobody to steal from you, and even if there was you'd eventually figure out who it was since everybody knows everyone. In a big city, someone can be in and out without you ever knowing, and you would never find them again. Indeed, most of the city residents I met in NZ still locked their doors (the people mentioned above were in smaller villages), while when my family went on vacation to a rural vacation home (in the US) my mom felt comfortable enough to leave the back door unlocked.