Why did so many reform initiatives of the early twentieth century emerge in large cities?

People were close together and cities presented new problems.

Early 20th century reforms emerged in cities because many people were able to come together quickly and discuss issues.

The issues emerging were caused by the cities such as immigration, sanitation, crime, and factory conditions. These were new problems which the middle class now had time and places to meet to address during the early 1900s.

Emergence of new type of association and challenges

Cities around 1900 enabled new sorts of associations to be formed, because they become overpopulated by people from all walks of life. these posed different challenges as the cities attempted to reconcile different people, with different demands. 'Reform' was an attempt to reconcile these different agendas.