A friend of mine asked me one of those infamous "simple questions" that should have a "simple answer:"
"Why was Saint Francis de Sales moved from January 29th [in the old liturgical calendar] to January 24th [in the modern liturgical calendar]" [with seemingly no reason; he wasn't born on that day, he didn't die on that day, he wasn't beatified or canonized on that day]?
Simple answer: Playing shuffleboard with the Saints.
Fullest answer I can find: Now, I am not sure why it seemed necessary to move Saint Francis de Sales from January 29th to January 24th. However, I did find out that January 24th used to be for Saint Titus. Now he has been moved together with Saint Timothy (from August 22nd) to the 26th (which bumped Saint Polycarp to February 23rd), so that they follow Saint Paul's conversion on the calendar (just as they followed Saint Paul in real life, I assume). With January 24th freed up, I guess they wanted to move Saint Francis de Sales to that day because that is the day he was buried (even though he died in late December). Bumping Saint Polycarp to February 23rd then bumped Saint Peter Damian back two days from February 23rd to the 21st. There was nothing on February 21st in the old calendar, so that branch of bumping was settled. The Queenship of Mary was moved from May 31st to fill August 22nd as a "close" to the octave of the Assumption (even though we don't celebrate octaves anymore except for Christmas and Easter). Thus the end of the month of Mary needed to be filled so they plucked the Visitation from July 2nd to fill May 31st, and now July 2nd sits "empty."
Of course, I am assuming that is why the moved things to the days they did.
What is sad is that not only does all this calendar shuffling really break away from the liturgical Tradition of feasts, but that the liturgical "reformers" also put so much effort into it.
As to the actual reason why all this was felt necessary, I don't know (let the conspiracy theories begin).
So, I guess that sheds some light on the question, but of course it raises way too many others.
EDIT - As a possibility, it's interesting to see a post from an article by the late Msgr. Schuler of the Sacred Music periodical, via The New Liturgical Movement:
Worst of all was the determination on the part of those who wished to promote the reforms to outlaw immediately the old missal, as if they were fearful that it would be a competition to the new and perhaps even prove to be too powerful a competition with the result that the new books would not be accepted. After all, there had never been a grass roots ground-swell for the vernacular; the Catholic people had not demanded liturgical reforms; the reforms came chiefly from the clergy, and at that, only a small percentage of the world's priests or bishops. Imposition of the new, therefore, required in the minds of some the immediate destruction of the old. How many convents, schools and even parishes burned the old Missale Romanum of Pius V as if it were a bad book!----
Now, part of the conspiracy against the Mass of Pius V and the Latin language involved a confusing of priests and people. The effort was made, and still continues, to associate Latin exclusively with the Mass of Pius V. The altar versus populum is associated with the missal of Paul VI. Latin is called "old," and versus populum "new." The over-simplification causes error and misunderstanding.
Full article here:
Monsignor Schuler on old and new forms of the Roman Rite