What prevented the Prophet, peace and blessings be upon him, or those Arabs who were on their innate nature (fitra), from embracing Christianity or Judaism? And was that due to the distortion of these religions even before the advent of Islam?

Among the Arabs, some became Christian in the pre-Islamic era (Jahiliyyah), and some were not Arabs, like Salman al-Farisi. It appears that very few were upon the correct Christian religion. The Prophet (peace and blessings be upon him) worshipped according to the religion of Abraham, which indicates the scarcity of People of the Book in the Arabian Peninsula. The Arabian Peninsula was not devoid of Jews and Christians, and they were mentioned in pre-Islamic poetry. The Jews and Christians had fallen into misguidance; the Prophet (peace and blessings be upon him) said: "And indeed, Allah looked at the people of the earth and detested them, both their Arabs and their non-Arabs, except for remnants among the People of the Book." They were aware of this, as in the story of Zayd ibn 'Amr ibn Nufayl, the Jews and Christians refused for him to follow their religion until he took his share of Allah's wrath and curse. So they guided him to the upright religion of Abraham, who worshipped none but Allah.