Because I am slender, bibs keep the shorts from moving around. Like! But I dislike the straps, they are uncomfortable and get moreso on a hot day. There is a solution to that, I have what Voler calls a Roadsuit which is like a skinsuit with a full length zipper so the top holds the shorts up via a seam around the sides and back. They are currently pricey, but I got one cheap as part of our club discount with Voler. I think other manufacturers are jumping into this product.

Voler: Velocity Roadsuit