Once upon a time, it was obvious what towns and small cities did: they served as central places serving a mainly rural population engaged in agriculture and other natural resource-based activities. The rural population was dispersed because arable land and other resources were dispersed, and so you had lots of small cities dotting the landscape.
Over time, however, agriculture has become ever less important as a share of the economy, and the rural population has correspondingly declined as a determinant of urban location. Nonetheless, many small cities survived and grew by becoming industrial centers, generally specialized in some cluster of industries held together by the Marshallian trinity of information exchange, specialized suppliers, and a pool of labor with specialized skills.
What determined which industries a small city developed? In some cases particular features of the location and nearby resources were important, but often it was more or less random chance at first, then a sequence in which one industry created conditions that favored another. ...
Obviously, this was a chancy process. Some localized industries created fertile ground for new industries to replace them; others presumably became dead ends. And while a big, diversified city can afford a lot of dead ends, a smaller city can’t. Some small cities got lucky repeatedly, and grew big. Others didn’t; and when a city starts out fairly small and specialized, over a long period there will be a substantial chance that it will lose enough coin flips that it effectively loses any reason to exist.
I’m not saying that there weren’t patterns of success and failure. Small cities were and are more likely to fail if they have miserable winters, more likely to come up with new tricks if they’re college towns and/or destinations for immigrants. Still, if you back up enough, it makes sense to think of urban destinies as a random process of wins and losses in which small cities face a relatively high likelihood of experiencing gambler’s ruin.
Again, it was not always thus: once upon a time dispersed agriculture ensured that small cities serving rural hinterlands would survive. But for generations we have lived in an economy in which smaller cities have nothing going for them except historical luck, which eventually tends to run out. ...
In the modern economy, which has cut loose from the land, any particular small city exists only because of historical contingency that sooner or later loses its relevance.