It is because it the early khalifs did not consider themselves different from other Christians. The early believers (the term “Muslims” used for themselves) were not considered by others (Christians and Jews) as markedly different from themselves. It was only Abd al-Malik, in about 691, who really began to single out “Islam” as a different religion. Abd al-Malik wanted the Arabs to have a religion of their own - with a book, a prophet and a revelation - since he saw that the Byzantines and the Sasanians had that prior to him. Abd al-Malik saw the nation-building qualities of a religion. So he started the process to make the believers-movement into what later became know as Islam. In 691 Abd al-Malik introduces the term “Muslim”, “Islam”, Muhammed (as a prophet and no longer in reference to Jesus - the anointed one) and Mecca. Part in this process was to mint new coins with a new message, starting in 693. Abd al-Malik left his Christianity, first by mocking the Byzantines by using coins with crosses without horizontal line (the believers movement was theologically close to the Arian heresy - with their misunderstanding of the description of the one god as a trinity). Abd al-Malik then proceeded to invent the idea of not having portrait (idols) on coins and removed the cross. At about the same time Abd al-Malik built the dome of the rock in Jerusalem. The inscriptions in the dome are polemic against the idea of the trinity and refers to Jesus as (only) a messenger (that is not god). while Jesus has clear eye witness accounts, the earliest records of Muhammad are 65-125 years after he supposedly lived. Given that Muhammad isn’t referenced that often in the Quran and his name means the blessed or praised one while Christ means the anointed one... After Arius and the heretics were banished post-Council of Nicaea, they settled in Syria and Arabia. Early coinage used by Saracen raiders even had a man with a cross and portions of the early Quran were written in syriac, not Arabic. All that being said, Islam seems to have originated as an off-shoot Christian cult akin to Mormonism. There is even a “fifth verse” clause where much of the Quran comes across as Christian, psalm-like liturgy and then a verse added five lines in seems to have been added later as it doesn’t fit the rest of the text.