Why Today is NOT Pius XI's Feast of Christ the King

The 'Solemnity' today and the Feast introduced by Pius XI are entirely different feasts, with different theologies and different prayers.

From Catholicism Coffee

For less than fifty years, the Feast of Christ the King occupied its former place at the end of October. Then, in 1969, as part of the liturgical reforms following the Second Vatican Council, Pope St. Paul VI issued the motu proprio Mysterii Paschalis, which ordered the reorganization of the liturgical year according to the new Calendarium Romanum, to go into effect the following year. Among a number of noteworthy changes to the universal calendar was the movement of Christ the King to a different date.

Ultima dominica per annum fit sollemnitas D(omini) N(ostri) I(esu) C(hristi) universorum Regis, loco festivitatis institutae a Pio Papa XI, a(nno) 1925 et ultimae dominicae octobris assignatae. Hac ratione, momentum quoque eschatologicum huius dominicae meliore in luce ponitur. — Calendarium Romanum (1969)

In place of the feast instituted by Pope Pius XI which was assigned in the year 1925 to the final Sunday of October, the solemnity of Our Lord Jesus Christ the King of the Universe occurs on the last Sunday throughout the [liturgical] year. For this reason, the eschatological importance of this Sunday is also better placed in the light. — translation mine

There is so much happening here! The reason for the feast is now deemed an eschatological one rather than a temporal one, the celebration is given a slightly different name (the former feast did not include universorum in the title), and the date is changed to the final Sunday before Advent. It is arguably not only a shift in the timing of the feast, but a new feast itself.