First, it should be pointed out that forms of slavery have been around for nearly 4000 years. According to one source, there are more humans enslaved today--in the 21st century!-- than at any time in history. Slavery was still widespread throughout many parts of the globe during the 18th century, so it should not be too surprising that it became an important part of developing the American colonies--an area of the world that was still in its relative infancy. The bottom line is that it was far cheaper and more profitable for plantation owners--and, for that matter, shop owners or private citizens--to purchase slaves to do their work than to hire a free man. Much of the labor required of slaves (such as picking cotton) was hard, back-breaking work. The one-time purchase of a slave, who could then father multiple siblings who would also become property of the owner, could provide untold dividends for generations to come.

As previously stated, there are many reasons that account for the growth of slavery in America.  On one hand, the need to pick and process cotton became of vital importance in the South.  Eli Whitney's invention of the cotton gin helped to make this process easier by processing the cotton with greater ease.  As more cotton was being produced, more money was being generated.  As more money was being generated, this money was used to buy more slave to pick more cotton.  In the end, this caused slavery to become an entrenched part of the Southern economy and life as it demonstrated how slavery was an embedded part of the Southern way of life.

Regarding slavery in colonial North America: There was a market for tobacco and(in South Carolina) rice, and there was lots of cheap land on which to grow them, but no dependable source oflabor to work the land and produce the marketable goods.Pohnpei397's first reason given is correct.  His second reason needs a bit of correction, that is to say, some authorities do not entirely agree with him:  Actually plantation owners got more work out of indentured servants for the time that they owned them, than out of permanently enslaved servants, because they drove the indentureds harder knowing that they would not have them for life but for a few years only.  See Morgan, "American Slavery, American Freedom" on this.  The problem with indentureds was: not enough of them and they often ran away and were hard to detect and recover once they got beyond the community in which they were known to be indentured.

Another answer states that slaves could be completely controled by their masters: No more is that true of slaves than of any other human employees except as to physical location; slaves slowed down their work, attempted to deceive their masters, broke their work tools, hid their work tools, stole food from storage buildings, ran away to the swamps for a two-week vacation, and did all of the other things that modern employees do.  Also, indentureds could be sold and their period of indenture could be extended rather (not completely) arbitrarily; if an indentured ran away and was recapured, the period of indenture was extended; also for various forms of resistance or disrespect to the master, the period of indenture could be extended.

As for one answer, that breeding houses were kept where strong male slaves were used as studs to breed more slaves, I don't know that this is false, but in a graduate course "The Old South", an undergraduate course "The South in History and Literature," and in several other books on the Old South that I have read, I have never even seen such a thing suggested.  It is true that many owners did not encourage marriage by slaves; those owners' slaves mated randomly.  Other owners did encourage marriage of slaves, but maintained the same right they had over their own daughters, of approving the slave's selection of a marriage partner before the marriage could take place.

I have strayed off of your topic.

Not only were there laborers for a lifetime with African slaves, but these slaves would reproduce and the children became slaves, costing the plantation owners much less than the price of a slave on the market. 

Considered property, rather than people with certain rights, slaves could be controlled by their owners.  For instance, if there were problems with a slave, there were no indenture papers, etc., and the owner could simply sell the slave.  Also, some slave owners actually had breeding houses (one still stands just over the Kentucky border in the southernmost tip of Illinois) where they had strong, big males produce children for them who would be hardy and strong themselves--for free.

Because of the semi-tropical climate of the Deep South, the dark-skinned African slaves were able to better handle both the temperatures and the tremendous exposure to the sun, which no British-born person could tolerate for long. (After 1680, there was a sharp decline in white indentured servants, anyway.)  Slaves were in the fields sometimes from sun-up to sundown. 

I agree with pohnpei's post.  In addition to those reasons, southern geography favored plantation cash crops like tobacco, indigo and rice. There was a great natural river system, favorable climate and fertile soil.  What's more, the cash crops they had chosen were labor intensive by nature, and there were simply not enough indentured servants to do the job, while Native Americans simply ran away.  The Spanish and Portuguese were already profitably using slaves in their colonies in Latin America, so it was inevitable the British would follow suit.

Charleston grew into a major slave importation point, as the southernmost port in the colonies at that time, close enough to Barbados in the Caribbean, so the slave trade itself was profitable and economical.

There are a number of reasons for this.  To me, the big ones are:

• I would say that the most important one was that the plantation system could not, in those days, function without forced labor.  The plantations were very profitable and were therefore attractive things to own, but they could not find paid labor very easily.  Free laborers would not stay very long (if they signed on at all) when they could easily get their own land and be their own bosses.
• White indentured servants were much less convenient and economical.  They had to be freed after a few years.  More importantly, they were much more likely to have a sense of their rights as Englishmen and be much less tractable than black slaves were.

I think these are the two big ones...