I don’t know about the possible history of the Annunciation as a day of obligation, but its nonobservance as such in the present day has nothing to do with this year’s accident of falling on a Sunday in Lent - the Annunciation is simply not a holy day of obligation:
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.