Why does the Qur'an address "people" and not "believers" when mentioning Hajj, Makkah, and the Kaaba, even though other religious obligations and discourses pertain specifically to believers?

The address "O humankind" is used in 20 places in the Holy Qur’an. This address is general, encompassing all human beings, believers and disbelievers alike, calling them to ponder the Hereafter and reminding them of Allah's Lordship. As for the association of the general address to "humankind" when Hajj and its rituals are mentioned, it is because the first time Hajj was enjoined, it was through the call of Abraham (peace be upon him) to all the people of the earth to visit the Sacred House of Allah.