Why is our religion called "Islaam"

Because one who enters it submits (Aslama) himself to Allaah, and he surrenders to (Istislam), and abides by, all of the judgements that come from Allaah and from His Messenger Salallaahu alayhi wasallam. Allaah Almighty said:

And who turns away from the religion of Ibraheem except him who fools himself?[Baqarah 2:130]

He also said:

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 all that exists."[Baqarah 2:131]

But whoever submits his face (himself) to Allaah and he is a good-doer then his reward is with his Lord [Baqarah 2:112]