why and how did large cities become what one commentator called "seedbeds for reform" in the nineteenth century?

Because of the various technological and industrial advancement, there was huge growth of cities in America.

Explanation:

Many new possibilities if employment were opened in America which allowed many immigrants to occupy in order to have a better livelihood. Transatlantic railroad construction companies observed Asians to be the cheapest form of labor and many Chinese were employed in the construction of it. Urbanization in America led to the growth of busy commercial cities like Boston, New York and Philadelphia.

Textile mills and many sweat shops attracted the women workers too. Although, cities sprang up to be developed during the nineteenth century, it also paved way for many reform movements like labor union movements, women suffrage movements.