Skilled indentures were able to sell their craftsmanship once they became freedmen, and therefore earned more income than the unskilled indentures who came before them.
Also, slaveowners could force their slaves to sign indentures of very long length (40 to 99 years), threatening them with sale elsewhere if they refused.
Indentures however prevailed in colonies that required skilled workers (in which the cost of training a slave was higher than the price differential between a slave and an indentured servant).