Because you have some unique interest. In a city you can fill it, not in a small town. Do you want a tank of Argon - you won't get that in the smallest towns (argon is useful enough that many farmers have a tank for welding). What about a new magic trick - order from amazon and hope it isn't cheap junk, or buy from the local store and see it in person first - you need a larger city to support that store. Maybe you want see a Broadway type musical, the larger the city the better talent they can afford to act. (in small towns the main actor is still pretty good but the support actors are not very good)