Perhaps tradition - purses have generally opened that way if they have a flap.
Perhaps convenience - carrying a bag with the clasp against the body might be uncomfortable and damaging to garments.
Perhaps because they are often used only for safe storage of gear when transporting rather than for being "worked out of". The only bag of mine that I work out of is a Domke and I often flip the lid "inside out" (outside in, really) to make the bag open and to make the gear accesable. Msotly, though, I set it down when working rather than walking with it on my shoulder.