It is called so because whoever reverts to Islam must submit to Allah (Exalted be He) and to all rulings introduced by Him (Exalted be He) and His Messenger (peace be upon him), for Allah (Exalted be He) says:
And who turns away from the religion of Ibrâhîm (Abraham) (i.e. Islâmic Monotheism) except him who befools himself.
To His saying:
When his Lord said to him, "Submit (i.e. be a Muslim)!" He said, "I have submitted myself (as a Muslim) to the Lord of the ‘Alamîn (mankind, jinn and all that exists)."
Moreover, He (Exalted be He) says:
Yes, but whoever submits his face (himself) to Allâh (i.e. follows Allâh’s Religion of Islâmic Monotheism) and he is a Muhsin then his reward is with his Lord (Allâh)