Well I could t think of a couple reasons.

1. 3D applications cost money and so do the computers to run em (my guess is that they don't want to assume you have those tools). A pencil and sketchbook is easy to get your hands on, and its easy for them to flip through.

2. I guess it shows if you have art in your background. Place spotlight on a poly sphere sitting on a plane in Maya and it'll look the way it should, recreating that in your sketchbook takes some know how.

Their reasons could be something totally different but thats what comes to mind when I read your question.