The world population has tripled in my lifetime, with the majority of that going to cities.
Most major cities are on rivers, and rivers are prone to flooding, so more are at risk, whether due to land use policies, lack of adequate flood-control policies or increased precipitation (due to global warming or cooling or too much concrete or house roofs or whatever).
True, most cities are on rivers, but also cities have more funds for public works like drainage systems to mitigate flooding than rural areas do. Most of the catastrophic floods I have seen have been around rural floodplains, not in cities (excluding silly below-sea-level New Orleans, that is).