Because there is no reason to screw with common terminology to begin with.

Neither Master nor Slave is a derogatory word. And using the concept of slavery to describe a relationship between inanimate objects in a technical context is not an endorsement of such practices between humans.

Because the master forces the slave to do it's bidding.

But ... so what? For one thing, the metaphor doesn't have to go that way. For another, "leader/follower" captures that too.

Not if the replica is pulling changes from the primary.