That's like asking why did Christianity start in Jerusalem?
The main religion of Saudi Arabia is Islam
Saudi Arabia's primary religion (along with most of the Middle East) is Islam.
Islam, since being a citizen of Saudi Arabia legally requires you to be a Muslim.
Islam started in Arabia 1432 years ago _______________________________________________________ It was the immigration of prophet Muhammad (PBUH) from 1432 years ago and not the religion start. The religion that had begun in the currently called Saudi Arabia is the full submission to God and surrender to His will (called in Arabic Islam). This is the religion since start of mankind in the area from the prophet Adam, Noah, ..., Abraham, ..., Moses,..., Jesus, and Muhammad (peace be upon them).
In Arabia the city of Mecca which is part of Saudi Arabia.
Saudi Arabia
Islam is the main religion of Saudi Arabia.
There is only one kind of Islam religion and it did start in Saudi Arabia and Arabs were the first Muslims
The last holy Prophet Hazrat Muhammad (May peace and blessings of Allah Karim be upon him) was born in Makka (Mecca) in Saudi Arabia. He started preaching Islam at the age of forty in Makka.
Saudi Arabia
Islam
Saudi Arabia Kaaba