Why do Catholics close the Eucharist off to other Christians that agree on the essentials of faith?

Irenaeus, in a letter addressed to Victor, the Bishop of Rome, pleads with him to him to not cut off unity on the Asian dioceses that celebrated Easter with a different tradition: > For the controversy is not merely as regards the day, but also as > regards the form itself of the fast. For some consider themselves > bound to fast one day, others two days, others still more, while > others [do so during] forty: the diurnal and the nocturnal hours they > measure out together as their [fasting] day. And this variety among > the observers [of the fasts] had not its origin in our time, but long > before in that of our predecessors, some of whom probably, being not > very accurate in their observance of it, handed down to posterity the > custom as it had, through simplicity or private fancy, been > [introduced among them]. And yet nevertheless all these lived in peace > one with another, and we also keep peace together. Thus, in fact, the > difference [in observing] the fast establishes the harmony of [our > common] faith. And the presbyters preceding Soter in the government of > the Church which you now rule — I mean, Anicetus and Pius, Hyginus and > Telesphorus, and Sixtus — did neither themselves observe it [after > that fashion], nor permit those with them to do so. Notwithstanding > this, those who did not keep [the feast in this way] were peacefully > disposed towards those who came to them from other dioceses in which > it was [so] observed although such observance was [felt] in more > decided contrariety [as presented] to those who did not fall in with > it; and none were ever cast out [of the Church] for this matter. On > the contrary, those presbyters who preceded you, and who did not > observe [this custom], sent the Eucharist to those of other dioceses > who did observe it. And when the blessed Polycarp was sojourning in > Rome in the time of Anicetus, although a slight controversy had arisen > among them as to certain other points, they were at once well inclined > towards each other [with regard to the matter in hand], not willing > that any quarrel should arise between them upon this head. For neither > could Anicetus persuade Polycarp to forego the observance [in his own > way], inasmuch as these things had been always [so] observed by John > the disciple of our Lord, and by other apostles with whom he had been > conversant; nor, on the other hand, could Polycarp succeed in > persuading Anicetus to keep [the observance in his way], for he > maintained that he was bound to adhere to the usage of the presbyters > who preceded him. And in this state of affairs they held fellowship > with each other; and Anicetus conceded to Polycarp in the Church the > celebration of the Eucharist, by way of showing him respect; so that > they parted in peace one from the other, maintaining peace with the > whole Church, both those who did observe [this custom] and those who > did not. Contained within the previous quote, there is a statement mentioning earlier presbyters that had diverse customs that remained in communion and shared in the Eucharist despite these differences. In 1964 the Second Vatican Council published the Decree on Ecumenism, Unitatis Redintegratio , where this statement is made: > Even in the beginnings of this one and only Church of God there arose > certain rifts,(19) which the Apostle strongly condemned.(20) But in > subsequent centuries much more serious dissensions made their > appearance and quite large communities came to be separated from full > communion with the Catholic Church - for which, often enough, men of > both sides were to blame. The children who are born into these > Communities and who grow up believing in Christ cannot be accused of > the sin involved in the separation, and the Catholic Church embraces > upon them as brothers, with respect and affection. For men who believe > in Christ and have been truly baptized are in communion with the > Catholic Church even though this communion is imperfect. The > differences that exist in varying degrees between them and the > Catholic Church - whether in doctrine and sometimes in discipline, or > concerning the structure of the Church - do indeed create many > obstacles, sometimes serious ones, to full ecclesiastical communion. > The ecumenical movement is striving to overcome these obstacles. But > even in spite of them it remains true that all who have been justified > by faith in Baptism are members of Christ's body,(21) and have a right > to be called Christian, and so are correctly accepted as brothers by > the children of the Catholic Church.(22) The Magisterium admits that there are "brothers" in the faith, as they share in the essentials of the faith. Despite this admission, the Catholic church practices closed communion where they do not offer the Eucharist to these fellow "brothers". How does the Catholic church interpret Irenaeus' more ecumenical approach to sharing in the Eucharist? I am aware that there are many other denominations that practice closed communion, but I'd like to narrow this down to a Catholic response.