I think a big piece of that was that the physical learning materials let you skip over stuff, but you still had to lug around the skipped-over stuff, so you never really stopped being reminded that it was there, and probably were able to return to it should it suit you.

(Of course, I also hold the opinion that the best learning materials are pretty modular, and very clear about what you're getting, and those go hand-in-hand. I think most things these days are not clear enough about what they are, and that's a problem.)