Southern plantation owners in the colonial era could not depend on using in dentured servants for labor because ?

There was an uncertain supply of workers.

Explanation:

Before slavery became established in America, indentured servitude was the most favoured system to obtain cheap labor. Indentured servants were usually poor immigrants who provided free labor for a number of years in exchange for shelter and food. At the end of their contract, they were granted a plot of land and were able to become farmers. However, the supply of workers that arrived in this way was uncertain, and as the need for labor increased, other methods were employed, particularly slavery.