> but the most convincing evidence that it was very hard was that early mines and factories were easily able to recruit all the workers they needed,

They may have needed the money but they sure didn't like it, at least here in the US. There was a large change of working for a wage, on a clock, compared to working on a farm or for a wage, and was a sizable violation of the Jeffersonian ideals of freedom. As to why people left, i dunno. There's probably a history research effort in it.