Why did Allah, the Most Blessed and Exalted, not say, "And proclaim among the believers the Hajj," just as He expressed Himself regarding prayer, fasting, and zakat by saying, "O you who have believed"?

The command to establish prayer and give zakat (definition) is repeated in the Quran, such as in His Almighty saying: "And establish prayer and give zakat and bow with those who bow" (Quran 2:43), and His Almighty saying: "And establish prayer and give zakat and lend to Allah a goodly loan" (Quran 73:20). Similarly, fasting was mandated by His Almighty saying: "Fasting has been prescribed for you" (Quran 2:183), and Hajj by His Almighty saying: "And [due] to Allah from the people is a Hajj to the House - for whoever is able to find thereto a way" (Quran 3:97). All these phrases indicate the obligation of these acts of worship, and the different styles do not alter their obligatory nature. Fasting and Hajj are obligatory upon Muslims. The Quran expresses the request for an action in various ways, such as a direct command, or by informing that the action is "prescribed" for those accountable, or "upon the people."