In the nineteenth century, in India, why do the male workers migrate to the cities alone and not with their families?

To minimise the costs of living in the city

Another new class within the cities was the labouring poor or the working class. To minimise the costs of living in the city, most male migrants left their families behind in their village homes. Life in the city was a struggle; jobs were uncertain, food was expensive, and places to stay were difficult to afford.
Yet the poor often created a lively urban culture of their own. They were enthusiastic participants in religious festivals, tamashas (folk theatre) and swangs (satires) which often mocked the pretensions of their masters, Indian and European.