According to the passage, during the 18th and 19th centuries cities were small in size mainly because

The first era in American urban history extended from the early 17th century to about 1840.Throughout those years, the total urban population remained small and so were the cities. At the first federal census in 1790, city dwellers made up nearly 5. 1% of the total population and only 2 places had more than 25,000 inhabitants. Fifty years later, only 10.8% of the nation’s population fell into the urban category and only one city — New York — contained more than 50,000 people. Largely because of the unsophisticated modes of transportation, even the more popular places in the early 19th century remained small enough that people easily walked from one end of the city to the other in those days.
    Though small by modern standards, these walking cities, as it were, performed a variety of functions in those days. One was economic. Throughout the pre-modern era, this part of the urban life remained so overwhelmingly commercial that almost every city owed its development to trade. Yet city dwellers concerned themselves not only with promoting agricultural activities in their rural areas. They also collected and processed goods from these areas and then distributed them to other cities. From the beginning then and increasingly in the 18th and early 19th centuries, cities served as centres of both commerce and simple manufacturing.
    Apart from the economic functions, the early cities also had important non-economic functions to play. Since libraries, museums, schools, and colleges were built and needed people to go there to visit or to study, cities and the larger early towns, with their concentrations of population, tended to serve as centres of educational activities and the points from which information was spread to the countryside. In addition, the towns with people of different occupational, ethnic, racial and religious affiliations became focuses of formal and informal organizations, which were set up to foster the security and to promote the interests and influence of each group. In these days, the pre-industrial city in America functioned as a complex and varied organising element in American life, not as a simple, homogeneous and static unit.
    The vitality of these early cities was reinforced by the nature of their location and by the process of town spreading. Throughout the pre-industrial period of American history, the cities occupied sites on the eastern portion of the then largely undeveloped continent and the settlement of countryside generally followed the expansion of the towns in that region. The various interest groups in each city tended to compete with their counterparts in other cities for economic, social and political control, first of nearby and later of more distant and larger areas. And always there remained the undeveloped regions to be developed through the establishment of new towns by individuals and groups.
    These individuals and groups sought economic opportunities or looked for a better social, political or religious atmosphere. In this sense, the city still helped the development of the successions of urban frontiers. Well, this kind of circumstance made Americans one of the most political and self-conscious city-building peoples of their time. It did not result in a steadily urbanizing society in the sense that decade by decade an even larger proportion of population lived in cities. In 1690, an estimated 9—10% of American colonists lived in urban settlements. A century later, that is, the end of the 18th century, though 24 places had 2, 500 persons or more, city dwellers accounted for only 5.1%of the total population. For the next 30 years, the proportion remained relatively stable and it was not until 1830 that the urban figure moved back up to the level of 1690. In short, as the number of cities increased after 1690, they sent larger numbers of people into the countryside. Then they returned. Nonetheless, the continuous movement of people into and out of the cities made life in the many but relatively small places lively and stimulating.