Why did Muhammad call his religion Islam?

Answer 1

To understand the answer the word and its meaning needs to be understood.

the word Islam, in its Arabic meaning is a description of an action one takes.

so Islam is the act of person understanding the purpose of creation and ultimately its own purpose as revealed again by all of our God= Allah.

Salam = peace and

Islam = acquiring peace through knowledge, accepting it, and devoting your life to it.

Most religions are named after a specific peoples or prophet and Islam was named without race, gender, ethnicity, etc... its for everyone.

The Prophet Mohamed Peace be Upon Him, is the last prophet of many including, Noah, Moses, Jesus, etc... Peace be Upon all of them.

Answer 2

It was not Muhammad (SAW) who called it Islam. It is Almighty God Who called it Islam. It is mentioned in the holy Qur'an while mentioning the events of the life of Hazrat Abraham (May peace be upon him). It is the religion that has been preached by all the Prophets sent by Almighty God for the guidance of humans. It is a continuity of Judaism and Christianity.
__________________________________________________________Islam is the religion of Allah (or God in English and same God worshiped in Judaism and Christianity) since start of universe creation. It is God religion that means full submission to God. Refer to question below.