I also suppose they wanted to get rid of filler, for example "Thank You For The Music" is nice onstage but adRAB nothing to the story, "One Of Us" isn't really needed as Donna sings The Winner Takes It All already, and Under Attack I would guess would not work nearly as well on the big screen as it does on stage.
Abba's USA hits catalogue isn't really that different to the UK one, so I suppose less well known songs like "Our Last Summer" "Slipping Through My Fingers" and "Honey Honey" that were never hits in the UK are there more because they fit in well with the story.