It would seem that the Feast of the Annunciation (March 25) would be at least as important as Christmas, if not more so, because it celebrates the moment Christ was incarnated as man, which would seem to be a greater mystery than his coming out of Mother Mary's womb, which is still great because it maintained her virginity and manifested Christ more visibly to the world. Why is not the Annunciation even a holy day of obligation anymore? The Third Council of Baltimore abrogated it in 1884 as a holy day of obligation in the U.S. Why?
I won’t speculate on why, but I will note that it is no longer a Holy Day of Obligation in the universal calendar.
Can. 1246 §1. Sunday, on which by apostolic tradition the paschal mystery is celebrated, must be observed in the universal Church as the primordial holy day of obligation. The following days must also be observed: the Nativity of our Lord Jesus Christ, the Epiphany, the Ascension, the Body and Blood of Christ, Holy Mary the Mother of God, her Immaculate Conception, her Assumption, Saint Joseph, Saint Peter and Saint Paul the Apostles, and All Saints.
It's one of the 12 Great Feasts of the Byzantine Churches (as close as we get to HDOs)--so important that if it falls on Good Friday (as it did a few years ago), the Divine LIturgy is celebrated.
One of the challenges the Councils of Baltimore had to deal with were the variety of customs involving fasting and holy days of obligation that immigrating Catholics had brought from their homelands.
If the average faithful Catholic is not aware of the great significance of Annunciation, I don't think making it a HDO will improve his/her attitude towards it.
Before someone asks, here is the next section:
Can. 1246 §2. With the prior approval of the Apostolic See, however, the conference of bishops can suppress some of the holy days of obligation or transfer them to a Sunday.
In the US, it seems only these are celebrated as Holy Days of Obligation (as I don’t ever recall having to go to Mass for the feast of St. Joseph or Sts Peter and Paul):
Holy Mary the Mother of God
Epiphany (transferred to Sunday)
Ascension (in some dioceses, transferred to Sunday)
Body and Blood of Christ (transferred to Sunday)
Nativity of the Lord
I do wonder if the fact that the feasts of St Joseph (Mar 19) and the Annunciation (Mar 25) are less than a week apart might be reason not to have both as Holy Days of Obligation (although, if only one were to be a HDO, I’d have chosen Annunciation).