Question

why was the slave dynasty called thus

Answer 1

Because when Muhammad Ghori died in 1206, it was his slave, Qutb-ud-din Aibak who became the ruler of the Delhi Sultanate. When Aibak died in 1210, it was his slave, Iltutmish who would become the next sultan.

Answer 2

because first king, qutub-din-aibak in this dynasty was a slave. he was made king as he was in the court of a king. after that king die, he was made the king

i'm not sure about this answer. comment if it is correct and forgive if it is wrong.