Lack of employment AND the terrible poverty which followed. It was move or starve. The following might help you understand.
"Irish seasonal and migrant workers were a highly mobile and flexible workforce, vital to Britain's booming 19th-century economy and economic revival after the Second World War. The traditional perception of the Irish navvy is by no means the whole story, but there were many who made a living in this back-breaking way.
In 1871, Thomas Ryan was a railway navvy living in the shanty town of Jericho, at Batty Green, near the Ribblehead viaduct in Yorkshire, and working on Midland Railway's Settle to Carlisle line. He had his family with him, and the birthplaces of his children, from Gloucestershire to Scotland, show how mobile they had to be. More than 2,000 navvies, mainly Irish and Scots, lived at Jericho between 1869 and 1875. "
http://www.movinghere.org.uk/galleries/histories/irish/working_lives/working_lives.htm