Question

why did some cities and towns decline in the 19th century

Answer 1

Around the same time, many smaller cities declined in importance. Many towns which were important manufacturing centres declined in importance because of a drop in the demand for what they produced. ... Machlipatnam, Surat and Seringapatam were some of the cities which were deurbanized during the nineteenth century.

Answer 2

In most parts of the Western world, modern cities emerged during and due to industrialisation. In Britain, for example, industrial cities  like Leeds and Manchester grew rapidly in the 19th and 20th centuries.

In the late 18th century, Calcutta, Bombay, and Madras rose in importance as  Presidency cities, centres of British power in India. At the same time, historically important ports and cities such as Machilipatnam, Surat, and Seringapatam declined.

The historic Mughal imperial city of Delhi also became a dusty provincial town in the 19th century before it was rebuilt as the capital of British India