Traditionally, why are 'pasta bowls' wide and shallow?
My hypothesis is that there is no functional reason for having a wide rim on a pasta bowl.  I contend that this is simply marketing, and the choice of the person presenting the meal. It is just a name for a bowl.  Pasta is served in all sorts of vessels; plates, deep bowls, shallow bowls.  Saucing, in part, determines the vessel.  For example, you can't serve tortellini in brodo on a plate.  Plates, bowls, and dishes are often designed and chosen for the way they compliment a final dish's visual appeal.  The Chinese (and other noodle eating cultures) don't necessarily eat noodles from wide-rimmed bowls, and I would confidently guess that you wouldn't have to look that far back to find a time that there was really was no such thing presented as a wide-rimmed, pasta bowl.  So, I would say that this is not a tradition at all.  This is a product of restaurants, ceramic makers, and the media (magazines and food TV).