RayFBecause the metal ones are formed from sheets of metal meaning that they have crevices and bends and rolled edges where stuff can accumulate and are harder to reach whereas these are injection molded so there are none of these.
RayFBenriners blow away the european style mandolines, I've used both in professional kitchens, and I guarantee you these seemingly dinky plastic ones are superior.
RayFThe huge metal ones drift and tend to need to be adjusted every few cuts, especially after they get a little older.
These are fixed blades with set screws for depth adjustment, they won't move unless you want them to. Also the blades are the most important part of any mandoline, and Benriners hold a very sharp edge for a very long time.
RayFThe only real downside is that Benriners don't have the wavy blades you need to make Gaufrettes (aka waffle fries/crosscut fries.)
And previously they only had the narrower sizes, but they've remedied this with the Jumbo range.
I'd advise to get the wider ones if you're in the market for a mandoline, it's not that much more expensive, and you'll be able to get larger potatoes and stuff sliced without having to trim it down.