Because of which reason the spouses of the poor, semi-illiterate and unskilled labourers migrating from rural areas are left behind in rural areas to look after children and elderly people?

Low wages.

In developing countries, poor, semi-illiterate and unskilled labourers migrating from rural areas frequently end up performing menial jobs at low wages in the informal sector in urban areas. Since wages are very low to support the family at the place of destination, the spouses are left behind in rural areas to look after children and elderly people.  Thus, the rural-urban migration stream is dominated by males.