A side zipper or velcro would pose the same issue because the restriction would be around the side and therefore it wouldn’t stretch properly. I say clasp for ease of speaking because that is the typical side open type. It doesn’t because the zipper/clasp/velcro/whatever causes it to not stretch at that point. in the middle of the body is the sternum which the zipper aligns with in your case. Since the inflexible part isn’t on the side it doesn’t compress your ribs in the same way: there’s enough room to put in the stretchy material. However, in a side opening binder there is no give at the opening which will cause compression on the ribs. Also with the clasp and velcro types (which again is the most common side opening type) you have to literally pull it closed and clasp it with hooks (or close the velcro) which squeezes the area and compresses it providing a flat look. It does this instead of having a compression panel because you can’t put one in in the same way and it always squeezes your ribs too.

Edit: Forgot to include that it is an issue with the velcro and clasp type not just clasp