It's about the balance of power. If abusers recognize that they have power over you, then they will abuse you.

If you seem emotionally incapable of protecting of yourself, then that is a sign that abusers can have power over you if they seize the opportunity.

In some cases though it could be literal or financial power. Your boss sees that you endure the abuse and yet keep coming back, and thus assumes that you must really need the job. Which means that your boss has the power.

Standing up for yourself with words is a good start, but without action it is pretty pointless. Sometimes you really do lack power in a situation, though, such as if you can't afford to quit the job.