If you search the forum you will find many threads on this subject and why they emigrated. The main reasons why people emigrated then are much the same as why people have moved to other parts of the UK and other countries in recent times. Such as higher wages and better working conditions and a chance of a better life. Others moved simply to a place where work was available.

Here is one of the threads:

Emigration of coal miners
http://www.forum.forest-of-dean.net/index.php?mode=thread&id=25364

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