The verse which settles the command to perform Hajj appears in Soorat Aal Imraan, where Allah says: “And Hajj to the House is a duty that mankind owes to Allah, those who can afford the expenses “ [Aal ‘Imraan 3:97]. And this part of this Surah was revealed in the year of delegations and is the evidence that Hajj was made obligatory in the ninth year of Hijrah.
The Prophet (peace be upon him) did not perform Hajj straight away in the ninth year of Hijrah. There were several reasons behind this including: A large number of delegations who came to him in that year. And this is the reason, that year is known to be the year of delegations. It was most important for him to receive the people coming to learn and know about the religion to Him PBUH, and indeed it was obligatory for the Messenger PBUH, to convey the message of the Almighty, to the people.