The system of slavery began to replace indentured servitude because
africans began to arrive in the colonies.
indentured servants began demanding higher “freedom dues.”
the need for a steady supply of low-cost labor increased.

the need for a steady supply of low-cost labor increased

Explanation:

In the beginning (18th century) before slavery, indentured servitude was a way for poor Euopeans to pay their way for the costly voyage across the Atlantic to the New Colonies.  Their contract terms were fixed in which they worked for their landowners or shippers until their debt was paid. As more and more of these indentured servitude served their contracts and became free, they also increased the demand for labor as they became landowners. The need for labor increased and the cost of indentured servants also increased.

White landowners, especially in early Virginia, turned to Africa for servitude. With time, using race to undermine Africans, greed for profits by landowners resulted in slavery and enactment of Slave Law in 1641.