Likewise, why the feast of St. Peter's chair in Antioch was suppressed in the Roman Church as a major feast.
It was a II class feast before Vatican II, and in the modern ranking it remains in the equivalent rank (feast). What changed, IIRC, was that it was not celebrated or transferred if it occurred on Sunday. I suppose it is probably observed in Rome no matter when it occurs.
I disagree, since demonstrably what changed was that it no longer is the feast of St. Peter's Chair in Antioch. In 1960, the January 18 feast celebrating the chair of Peter in Rome was moved to February 22, and the celebration of the chair of Antioch suppressed altogether. So the feast of St. Peter's chair in Antioch has no rank, since it no longer exists. February 22 is for the chair in Rome now.
My understanding has always been that there were two "Chair" feasts (18 January for Rome, 22 February for Antioch) and it stayed that way until after Vatican II. It seems, however, that, by 1955, the see commemorated on 22 February was changed, as you say, from Antioch to Rome. Without your post, I would not have gone through the books to search that out, so I thank you. In any case, even with this change, both feasts were still kept on the calendar with similar names but different Masses/Offices (January's is more heavy on "Petrine primacy", IMO, than February's, which I suppose reflects their original respective Roman/Antiochian character).
By 1960, the January feast was dropped, not transferred--even though the commemoration had already been switched from Antioch to Rome, the more "Roman" texts of January were not used to replace those of February. My presumption until now has been that they kept the February date because it is older: the Syrian Orthodox Church also observes it as the feast of the see of Antioch (this is why I erroneously believed that the RC's did not change the commemoration), and I have not yet heard it claimed that this is a Latinisation in our Church, so unless you have any information to the contrary, I think it is a reasonable guess. After Vatican II, it seems the feast retained, at least in its description if not in its texts, the association with Rome.
I find all of that interesting, but that wasn't the main focus of my comment, which focused more on how
the feast is celebrated rather than what
is celebrated. This discussion has helped flesh things out a bit for me and I find that helpful. Even so, I'm not sure it is accurate to say the feast of St Peter's Chair in Antioch "no longer exists". In terms of the post-Vatican II liturgy this is true, and it is true in terms of the naming of the feast in the liturgy for ~15 years prior to the reformed liturgy. But in terms of the texts of the Mass/Office of the pre-Vatican II liturgy (which remains as a legitimate form of the Roman liturgy to this day), it's a bit more complicated IMO. They could've switched the "Roman" texts for the "Antiochian" texts (transferring texts has been done before) or written a new Mass/Office (as they did for 8 December and 15 August), but they simply used "Antiochian" texts for a "Roman" commemoration. Make of that what you will, but since we also have the 22 February feast, I'm more inclined to think this was one of many "botched" liturgical reforms where some things got mixed up and even lost in the shuffle.