Indentured labour was done by contract for a fixed period, and they were paid. For people who believe that there was workers who got on the ships through their own agency this is important. This undoubtedly happened along with those who were compelled or deceived.

Frankly, I think the reason that it is not more often referred to as slavery is because it wasn’t officially referred to as such contemporaneously. But there is legitimate debate around the differences between Indian indentured servitude and slavery.