I'm not an expert or even close to that. My take is that older combination guns were built like this because that was the most common action available early on. There wasn't a lot to do to turn a sxs shotgun into a sxs combo gun. Husqvarna Model 17's were built in fairly large number considering the number available today.
With the beginning of the production of the O/U shotgun building on them became more prevalent, I think for a number of reasons. The rifle barrel set lower in the action, more in line with the hinge pin and locking lugs giving the locking mechanism greater mechanical advantage, stronger lock up without having to make the firearm more massive.
BRNO O/U combo guns are one of the few with the rifle barrel on the top but if you look at them the trunions are very high on the action and the locking bolt is even with rifle barrel. Savage 24 have the rifle barrel on the top but if you try and increase the power of them, rechamber a 30-30 to 30-40 they will pop open.
BRNO 305 12ga/5.6x52R(22 Savage Highpower)
By the way beautiful old combo gun.