- 1936 Cleveland, Great Lakes Exposition half dollar graded Mint State 68 by PCGS
I was a coin collector until 1991, so got to follow the ups and downs of coin grading from the beginning.
While there are a fair number of graded/slabbed non-US coins around, it's mostly for the benefit of US dealers/auctions. You still see British coin dealers grading coins the old-fashioned way, and they list coins with descriptions like "Good EF, NGC MS-63".
They realize coins (and stamps) are not commodities that can be sold sight-unseen. NGC also slabs ancient coins (they will pretty much slab anything you want to pay them to look at) which makes sense for authentication, but doesn't get you much of a price bump for grading.
My theory as to why coin slabbing caught on in the US, but stamp slabbing didn't, is that plastic holders actually make good sense as a way to store coins. Many people keep better coins in inert plastic capsules so you can look at your coins, and store them away neatly in a way they won't get damaged.
Aesthetically, they look better that way than in 2x2 holders. None of that is true for stamps, which do fine in albums/stockbooks/stockpages and look ridiculous in slabs.