Why were the words of the mystery of faith taken out of the consecration in the New Mass?

Pope Paul VI approved the removal of the words of the mystery of faith from the consecratory formula of the wine not to diminish belief in transubstantiation but to “express more clearly” the actual words spoken by Jesus Christ during the Consecration:

The words ‘mystery of faith’ . . . are not those of our Lord at the institution of the Eucharist. . . . None of the scriptural accounts of the institution records these words; they are not to be found in other formulas of Consecration recognized as valid by the Church; and hence they are not required for a valid Consecration. It is because these words are not among those spoken by our Lord that they have been rearranged in the revised Roman Missal containing the New Order of the Mass. ‘The words “Mystery of Faith” . . . taken out of the context of the words of our Lord and pronounced by the priest,’ Pope Paul VI explained in his 1969 Apostolic Constitution Missale Romanum, ‘serve as it were as an introduction to the acclamation of the faithful (James Likoudis and Kenneth Whitehead, The Pope, the Council, and the Mass, 113–114).

The Second Vatican Council had this to say about legitimate changes to the liturgy:

The liturgy is made up of immutable elements divinely instituted, and of elements subject to change. These not only may but ought to be changed with the passage of time if they have suffered from the intrusion of anything out of harmony with the inner nature of the liturgy or have become unsuited to it.

In this restoration, both texts and rites should be drawn up so that they express more clearly the holy things that they signify; the Christian people, so far as possible, should be enabled to understand them with ease and to take part in them fully, actively, and as befits a community (Sacrosanctum Concilium 21).