Why were English criminals brought to the American colonies?

The practice of deporting criminals to the colonies was seen as a humane way of dealing with criminal offenses and of saving the victims from the worse forms of punishment traditionally meted out domestically.
The practice must be seen in its historical context. In the 1600s and 1700s, Britain was wracked by civil conflict and by rebellions. Not only was there the English Civil War of the 1640s and '50s, but also the Irish Confederate Wars during the same period, as well as the Scottish uprisings of both this period and of the 1700s (i.e., the 1715 and 1745 rebellions after the Union Act in 1707 ended Scottish sovereignty, and many Scots wished to restore independence). In all these cases, prisoners were often exiled to America as an alternative to punitive measures in Britain.
In connection with this practice, one has to remember that the North American colonies were being regularly populated by religious dissenters, such as the Puritans in New England, Quakers in Pennsylvania and New Jersey, Roman Catholics in Maryland, and others. These people were not truly "criminals" in a legal sense, but represented an undesirable element the English ruling class wished to be rid of. To send actual lawbreakers to the colonies was simply another form of this exile of the "unwanted."
The English, of course, were not alone in this practice, as the French during the same period exiled criminals, including people such as common thieves and sex workers, to Louisiana. After the American Revolution ended and the deportations from Britain to America ceased, the British established the penal colony of Botany Bay, Australia in 1787 as a replacement venue.