Thank you both for your educated guesses. I’m pinging Mrs. Don-o - perhaps she can shed some light on this :-)
Here's an answer to that very question from Fr. Pat McCloskey. Here is another reflection from Fr. Raniero Cantalamessa.
It occurs to me that for people unable to come to church on Good Friday (for example, because of work schedules), Palm Sunday would be the only time they can hear a Liturgical reading of the Passion all year.