My impression from visiting a few shops like Chapuis, Gaucher, etc a few years ago is simply that there are two different markets for these DRs. the 7x65r, 8x57, and 9.3x74r rifles are made for "run of the mill" European big game hunters. Lots of these rifles sold and lots of them used. These "express rifles" have to compete with relatively low priced semi-autos, pump guns, lever guns, and straight-pill bolt actions for driven game hunters, so they cannot be priced too high. Hence the name Chapuis gives for their entry level gun: "Utility Grade Express, or UGEX". At least in France the price for the entry level express rifle is a bit lower than in the US, and the prices of entry level semi-autos, bolts, pumps, etc are a bit higher, so there is not that great a price difference between the two, so the double rifle is a viable choice. Also, in some European counties some of the other actions might not be legal (semi-autos, pumps, etc), so a medium calibre DR makes a lot of sense for driven game hunting.

Once you get above the 9.3 and start talking 375, 470, etc. All the makers know you are a guy who is either looking for an African rifle for DG, or (more rarely a shooter who likes big bores for the range). Hardly anyone in Europe buys these rifles unless going traveling hunting. I've never seen a 375 being used on a European hunt, the way you can see 375s in Alaska or Canada. These hunters will pay more for a rifle, since they can pay for going to Africa. So the makers can essentially ask for a higher price and someone who wants a big bore DR will pay it.

Also I think (just a guess) economies of scale go into it. They know they will sell far more 8x57r rifles than 470NE so the unit cost of each rifle goes down, etc.

All of this is just a mixture of my own speculation, and things told me by makers and gunshop owners, so I could be completely wrong.