Why do you think indentured servants became less common as slavery grew?

Indentured servants were suppose to work for 7 years, but often after a couple of years they would leave the indentured contract and faded into the population. When that happened they couldn't be separated from other people because they were white. The slave on the other hand would always be a slave no matter where he/she went because they were black. To leave a plantation they needed a pass and any white person could stop them asking to see the pass. Slaves were owned by another person and considered property an indentured servant was a worker under contract so it was easier to buy a slave for the same amount of money spent on the indentured servant. The slave couldn't escape easily and could be picked out because he/she was black.