Why didn't Ottoman empire control whole Arabian peninsula?

Prior to WW1, there was no Saudi Arabia; the Arabian Perninsula was divided into different regions such as Hejaz,Nejd, Yemen etc. The region unoccupied by the ottomans was called Nejd. read more

To the west, Ottoman Empire already controlled the holy cities of Mecca and Medina, hugging the Western coast. To the East, Ottomans controlled Basra and Mesopotamian region, establishing a strong control over the trade from Asia in to Europe without needing to take Oman. Otherwise, the Arabian peninsula is essentially desert, little of value. read more