Good question.
One reason is that for some flanged NE chamberings, brass is often expensive and not always perfectly made.
In Australia, quality cases in something like 450 Basic for eg has at times cost around $8 Aus each or $800 per 100.
Compare that to something like .375 H&H Belted Mag which is often well under $1 Aus or less than $100 per 100 for excellant quality cases.

Another reason is that sometimes VERY good deals can be found on high quality doubles chambered for rimless cartridges.
A certain Thys .458 Win Mag SxS comes to mind, owned by one of our members here.