Why is it that the ذكر (remembrance/mention) in the prayer is specific to the Prophets Ibrahim and Muhammad (peace be upon them both), to the exclusion of other prophets?

The Ibrahimiyah prayer (Salat al-Ibrahimiyah) came as an elucidation of Allah's command to invoke blessings upon the Prophet. In it, we say: "O Allah, send prayers upon Muhammad and upon the family of Muhammad, as You sent prayers upon the family of Ibrahim. Indeed, You are Praiseworthy and Glorious. O Allah, send blessings upon Muhammad and upon the family of Muhammad, as You sent blessings upon the family of Ibrahim. Indeed, You are Praiseworthy and Glorious." Its meaning is: Answer the supplication of Your angels who prayed for the family of Ibrahim by saying, "The mercy of Allah and His blessings be upon you, O people of the household," for Muhammad and his family, just as You answered it for the existing members of Ibrahim's household. It is permissible to invoke blessings upon prophets and companions dependently (as part of a larger supplication), and some scholars have also permitted it independently. It is preferable to invoke blessings upon prophets, angels, the Prophet's wives, his progeny, and generally upon people of obedience.