Why are only Muslims allowed to visit the holy city of Mecca, Saudi Arabia?

Why are only Muslims allowed to visit the holy city of Mecca, Saudi Arabia? - Mecca and Madinah are cities of great importance in Islamic tradition -- centers of pilgrimage and prayer, sacred places where Muslims are free from the distractions of daily life. The ban on non-Muslim visitors is mentioned in the Qur'an as follows: "Oh you who believe! Truly the idolaters are unclean; so let them not, after this year, approach the Sacred Mosque...." (9:28).

A sign near Mecca, Saudi Arabia indicates the exit for non-Muslims to take before entering the holy city. Huda, About.com Guide to Islam

This verse specifically refers to the Grand Mosque in Mecca; later scholars have included Madinah in this ruling as well. There are some Islamic scholars who would permit exceptions to this general rule, for trade purposes or for people who are under treaty permission. There is also some debate about the exact area and borders of the restricted area(s). The government of Saudi Arabia, which controls access to the holy sites, has decided upon a strict ban on both cities in their entirety.

Restricting access to Mecca and Madinah is intended to provide a place of peace and refuge for Muslim believers and preserve the sanctity of the holy cities. At this time, millions of Muslims visit the cities each year, and additional tourist traffic would simply add to the congestion and detract from the spirituality of the pilgrimage visit. ( about.com )