Because it happens to be the historical truth and our Church is a truth-telling institution.
The “perpetual virginity” of Mary is not “the historical truth”; it is “the historical belief” of your church. It cannot be proved by logic, evidence or science. There are no means to prove that “perpetual virginity” corresponds to the actual state of Mary and in fact, the contrary evidence from scripture and an extra-biblical source is more probative than the isegesis and tradition of your church.
That being the case your second statement “our Church is a truth-telling institution” fails since the “perpetual virginity” of Mary is a major tenet of your church.
Because it happens to be the historical truth and our Church is a truth-telling institution.
Truth telling is not usually connected with the Roman church.
The major forgeries sold as truth by Rome to grab power and fortune include Pseudo–Isidorian Decretals, A.D. The Donation of Constantine and the Liber Pontificalis.
John Huss actually believed Romes guarantee of "safe passage" until they lit the flames at his feet.