I could tell you, but they'd have to kill me.

Actually, it's just as much a mystery to me. I imagine the main cause is that the relatively short time these smaller 3-ring binders have been available has prevented anyone from accumulating many. And those collectors who did purchase are using them to hold album pages. Stamps dealers today may not stockpile them, either, preferring to wait until someone wants one to order from Scott so they don't spend too much money on inventory. The 3-ring option has not been around nearly as long as the old, reliable 2-post Scott binders which go back generations. My 2-post National album dates to the 1960s, almost 60 years ago, and they were around well before that. The 3-ring is relatively new.
Scott's fairly recent decision to move to the metal hinge was most likely necessitated by the high failure rate of cloth-hinged binders which was driving collectors to look for alternatives, themselves (Lighthouse 3-ring binders was one option). I wonder if they'll add metal hinges to their International binders, as well, as those behemoths really take a beating from their enormous weight when filled with pages. My cloth-hinged Scott binders are all fine, but apparently many collectors weren't quite as careful with theirs, and maybe they complained. So we get metal hinges.
'm a bit ambivalent about "armoring" up stamp albums like this. On the larger 3-ring Scott binder, the metal hinges are very unappealing. It makes the binder look like an industrial binder for steel mill invoices rather than a stamp album. But on the smaller 3-ring binder, the hinges may look less bad -- and "less bad" is about all you can hope for if you're offered industrial sturdiness in your stamp albums. I'll probably get used to it. I'm still trying to like the 3-ring binder itself which I'm not a big fan of.
All you can do is browse
ebay, I suppose. It is funny that Scott (or dealers) never had a 3-ring clearance sale, at least none that I know of. But again I'd guess there were't that many of the binders made in order to need to do that. I think one dealer, and I think it was Potomac Supplies, was discounting non-metal hinged 3-ring binders by a very small amount. You might check their website. Not sure about Subway Stamp Shop.