There were several factors which made the conquest of the Hijaz almost inevitable.
Husain bin Ali had appealed to the British for aid. The British responded by merely asking Ibn Saud to ensure that the safety of British pilgrims should not be compromised. Such a message, with no mention of Husain bin Ali, amounted to acquiescence in Ibn Saud's conquest of the Hijaz. It clearly indicated that the British would not attempt to intervene on behalf of the Shareef.
Ibn Saud, the ruler of a vast land and a growing population, needed a reliable source of revenues. Without adequate financial resources, he could not maintain his hold on the fickle bedouin on which, in large measure, depended the cohesion of his domain. Only the annual pilgrimage to Makkah could offer such a source of funds.
But neither Ibn Saud, nor the Muslim world at large, would have considered financial need as sufficient justification for the conquest of the Hijaz. There were other compelling reasons. For the followers of Ibn Saud, it was wrong that the Holy Cities should be bedecked with pictures and ornaments and that the sounds of music should be heard everywhere. The process of separating pilgrims from their money, which ranged from mere over-charging to merciless robbery, was an integral part of the Hajj and the Makkhan economy. The Shareef himself exacted high dues of the pilgrims, demanding that he be paid in gold, while the bedouin in the surrounding countryside robbed the Hajjis with impunity, the road between Makkah and Madinah being notoriously unsafe.
Husain bin Ali had proved himself an unworthy guardian of the Holy Places and, when he proclaimed himself Caliph, he had gone too far.