Because he is the first Monotheist, and is the ancestor of many monotheistic religions.
The common ancestor of the three religions Judaism, Christianity and Islam was Abraham
Abraham fathered these religions: Judaism, Islamic and Christianity
Abraham/Ibrahim
There are actually five classical monotheistic religions: Judaism, Zoroastrianism, Christianity, Islam, and the Baha'i Faith. The Prophet Founders of each of these Faiths was a direct descendant of Abraham.
Judaism, Christianity, and Islam.
Abraham's descendants became the Israelite nation, which later evolved into the Jewish people. Through his descendants, Abraham is considered the patriarch of three major monotheistic religions: Judaism, Christianity, and Islam.
Islam, Christianity and Judaism
The Abrahamic faiths are all monotheistic ones that have their roots in the prophet Abraham, the central religions are Christianity, Judaism, Islam and Bahá'í.
The patriarch of the monotheistic religions of Judaism, Christianity, Islam, and the Baha'i Faith is/was Abraham.
Jews say that Abraham is the father of their faith and the genetic ancestor of their people. Muslims say that Abraham was a prophet in their faith (though not the founder) and the genetic ancestor of the Arab people (who are now mostly Muslim), but not necessarily all Muslims. Christians consider Abraham to be their spiritual ancestor but do not consider him to have founded Christianity or to be the actual ancestor to most Christians.
By definition, Abraham (from whose name the term "Abrahamic" is derived) was monotheistic. He had one god. Therefore, all Abrahamic religions follow the god of Abraham, who has many names. They probably wouldn't be Abrahamic if they did anything else.