Why hold it in Riyadh? Probably for no bigger reason than it’s the capital and largest city of Saudi Arabia, and is also the seat of its government.
Why not hold it in Mecca? Probably for the simple reason that Mecca has no airport: the nearest airfield is over 40 miles away, at the King Abdulaziz International Airport in Jeddah.
(Plus, per the article, the Arab League and the OIC were having separate meetings in Riyadh anyhow, but they decided to combine them into one given current events.)
I wonder how the UN would react if Trump 47 were to host a worldwide Jewish/Christian Summit, where the people of the earth could decide what to do about the “Islamic problem”?