I suppose I could speculate that I remember it well because it was beautifully written and illustrated (which it was), or because I was young and impressionable (which I also was), but I think it was both of those things, combined with the fact that many, if not most, stories for younger children have got a happy feel to the book, whereas this one didn’t.
The dog-eared version I have (from 1983) by Michael Scott is full of wonderful references to old Ireland and the legends from long ago. The story is about 4 siblings who were changed into swans by their wicked step-mother. They were forced to live for 900 years on the lakes and seas of Ireland. The only thing she could not do was take away their voices and so they sang and talked in the story.