"Fr. Hunwike's Liturgical Notes" has the answer, which I had forgotten, if I ever knew it:
...surely one of the most crass examples of the Hermeneutic of Rupture incarnated in the Bugnini 'reforms' [was] ... when, with immense cynicism, the 'reformers' reduced July 1 to a feria on the flippant grounds that the Precious Blood would get a perfectly adequate 'covering' by being merely added to the title of Corpus Christi. Thus a nice piece of Pius IX liturgy disappeared: the memorial he placed on the calendar to commemorate his return to the City after the Roman Revolution of 1848. There is nothing vulgar, incidentally, about doing that sort of thing to the calendar, or, if there is, it is simply the vulgarity of an incarnational religion.Pius IX assigned the feast to the first Sunday in July, Pius X moved it to July 1 as part of his effort to put back into use Sunday Masses of the Roman Missal that were frequent impeded.