Lots of factors...
I know what you mean - I never thought I'd ever find one, either! Relative age and popularity of the machines has an influence. For example, the last of the 'slant front' style Laundromats were produced in the early 1960s and that's getting on for 50 years now. The machines were relatively popular, too, so I suspect that most owners used 'em till they were truly finished then scrapped them. It was not unusual to seem them still in service in the late 60s and early 70s and I will bet that as soon as WCI gobbled up Westinghouse, repair parts were discontinued so the remaining ones could no longer be maintained.
I know from a Canadian perspective that even into the early 60s, folks here tended toward the lower-priced and thriftier wringer washers. My parents arrived in Canada in 1954, got their first wringer machine in 1955 but did not have an automatic until 1964! Of course my father was pretty steamed that my mother went ahead and bought a brand-new washer and dryer without consulting him, but that's a whole other story...
But don't despair, they do turn up. Be prepared to do some searching outside of your home area, though. You may find a dream machine on the other side of the continent so you may have no option but to have it shipped (and this can get expensive). But you never know what you'll find once you start searching...
