What was one factor that led to more people seeking jobs in cities and factories?

immigration

Explanation:

in the us for example immigration brought in immigrants from other countries. many of them left everything behind and came to the us so that their families could have better lives. many of these same immigrants ended up arriving in cities in the us. needing money for their families, immigrants sought out jobs, and one of the most popular jobs for an immigrant to have was as a worker at a factory.

so more immigrants meant there was more people seeking jobs in cities and factories

The machines slowly took over most jobs. only some humans were needed