What was the primary pre-Islamic Arabian religion of Muhammad's (PBUH) time?
If you really want to know....here's some detail on it...
"Most of the Arabs had complied with the call of Ishmael (peace be upon him) , and professed the religion of his father Abraham (peace be upon him) . They had worshipped Allâh, professed His Oneness and followed His religion a long time until they forgot part of what they had been reminded of. However, they still maintained such fundamental beliefs such as monotheism as well as various other aspects of Abrahams religion, until the time when a chief of Khuzaa, namely Amr bin Luhai, who was renowned for righteousness, charity, reverence and care for religion, and was granted unreserved love and obedience by his tribesmen, came back from a trip to Syria where he saw people worship idols, a phenomenon he approved of and believed it to be righteous since Syria was the locus of Messengers and Scriptures, he brought with him an idol (Hubal) which he placed in the middle of Al-Kabah and summoned people to worship it. Readily enough, paganism spread all over Makkah and, thence, to Hijaz, people of Makkah being custodians of not only the Sacred House but the whole Haram as well. A great many idols, bearing different names, were introduced into the area. [Mukhtasar Seerat-ar-Rasool p.12]
An idol called Manat, for instance, was worshipped in a place known as Al-Mushallal near Qadid on the Red Sea. Another, Al-Lat in Taif, a third, Al-Uzza in the valley of Nakhlah, and so on and so forth. Polytheism prevailed and the number of idols increased everywhere in Hijaz. It was even mentioned that Amr bin Luhai, with the help of a jinn companion who told him that the idols of Noahs folk Wadd, Suwa, Yaguth, Yauk and Nasr were buried in Jeddah, dug them out and took them to Tihama. Upon pilgrimage time, the idols were distributed among the tribes to take back home. [Bukhari 1/222] Every tribe, and house, had their own idols, and the Sacred House was also overcrowded with them. On the Prophets conquest of Makkah, 360 idols were found around Al-Kabah. He broke them down and had them removed and burned up. [Mukhtasar Seerat-ar-Rasool p.13-54]
Polytheism and worship of idols became the most prominent feature of the religion of pre-Islam Arabs despite alleged profession of Abrahams religion."
Taken from :
Ar-Raheeq Al-Makhtum - Religions of the Arabs- SunniPath Library - Books