I don't know definitively what the Pope was thinking, but it seemed obvious to us that:
* Communism was perceived as a great threat in 1950.
* Communism's high holy day was May 1.
* Communism professes to be "for the good of the worker"
* St. Joseph is the "model of workmen" (to quote the St. Joseph litany)
* Catholics should turn to the saints for answers, and pray to them in their needs.
* If workers indeed have special needs in the 20th century, why not go to St. Joseph instead of the godless materialists for help and solace?
It's true that many Catholics were tempted by labor unions, socialism, communism, etc. because let's face it -- Capitalism as it exists today (and in 1950) is hardly a just, fair, Catholic institution.
Putting those things together, it's not surprising that they added another feast day for St. Joseph.
As for "why it wasn't our favorite feast" -- mostly because we didn't even sing the full Gregorian chant propers, because let's just say they can't compose good melodies like they used to. I marvel at the beauty of Chant, but I don't think just anyone can come up with melodies like that.
But we recognized the feast out of obedience, since we use the 1962 Missal. We're not the Church, and there is no just reason to reject things "just because we don't like them". There has to be a higher cause than obedience, if something is to be rejected -- such as Faith, Hope, or Charity. None of those is in jeopardy by celebrating the Feast of St. Joseph the Worker on May 1.
It's the same reasoning why we use the 1962 missal, and not the 1954, 1945, or 1920. Who are we to pick and choose? Archbishop Lefebvre basically "picked" the latest Missal that didn't begin to damage the Faith. We should always go with the Church, unless we have a serious reason not to. It's easy to forget today in this Crisis, but it's not a small matter to go off on one's own.
Matthew