People used to know their neighbors. They would watch out for each other, and potentially everyone's business was spread in hours. This kind of atmosphere created enough security that people felt no reason to lock their doors. The same is true for small villages elsewhere in the world, they don't lock doors and some stores trust the constomers to just take things and leave payment with nobody at the counter to oversee the transaction. Unfortunately, people use large populations to their advantage and will be more likely to commit crime if they can hide amongst a crowd, anonymous. As time went on, the population grew, people became more reclusive, and now they lock doors.