I have no idea why.
From the Wikipedia: Pop Goes the Weasel.
“Pop Goes the Weasel” is a common children’s song, numbered 5249 in the Roud Folk Song Index. The lyrics of the version I grew up with are as follows:
All around the mulberry bush
The monkey chased the weasel
The monkey thought it was all in fun
Pop! goes the weasel.
Those lyrics defy explanation. For one thing, there are not many areas of the world where monkeys, weasels and mulberry bushes coexist, and in none of them do people traditionally speak English. Mulberry trees grow all over the place: Europe, southern Asia, Central and South America, eastern North America and southern Africa, but there are no monkeys in eastern North America, unfortunately, and no weasels in southern Africa. The only non-human primates native to Europe are the Barbary apes.
Furthermore, the lone South America type of mulberry tree — morus insignis — mostly appears to exist near the Pacific coast, whereas South American weasels typically live inland.
Basically, the only places where a monkey could feasibly chase a weasel around a mulberry bush without human intervention are Mexico, parts of Panama, Ecuador, Colombia and Peru in the Western Hemisphere and China, Laos and Vietnam in the Eastern Hemisphere.
Moreover, though weasels do communicate vocally, they either gurgle or squeal or screech or purr, depending on the type of weasel and the situation, but never pop. And though a playful monkey might chase a weasel “in good fun,” it’s unlikely that monkeys ever eat weasels. In fact, tayras — a cousin to the weasel sometimes included in the larger weasel umbrella — are known to attack monkeys, so it may be more common for weasels to chase monkeys than the other way around. I know: I’m blowing your mind right now.
So the song makes no sense. But as it turns out, and somewhat predictably, the lyrics to “Pop Goes the Weasel” commonly sung in the United States are not the original ones. Our version is the remix.
The song dates back to at least the 1850s in England, but none of the known versions from that era contain references to mulberry bushes and only one verse mentions a monkey. The first time it included a monkey chasing anything, at least by the Wikipedia’s research, came in a version printed in Boston in 1858, when the monkey was chasing the people around the cobbler’s house.
It’s worth noting that the Wikipedia deems the song’s popularity on both sides of the Atlantic in the 1850s as a “dance craze.” No joke. Some 160 years ago, “Pop Goes the Weasel” was the hottest new jam at all the clubs. Newspapers called it “the latest English dance” and (inaccurately) credited it to Queen Victoria herself, and apparently started using the phrase “pop goes the weasel” as an expression outside the context of the song, like how people once claimed to be too legit to quit or, presumably, said, “c’mon, baby, do the loco-motion” to compel their friends into action at various times in the 60s, 70s and 80s.
It makes “Gangnam Style” seem palatable, really.
As it turns out, there are a variety of theories as to what “Pop Goes the Weasel” actually means, but it seems most likely it comes from Cockney rhyming slang — itself the subject of a reasonably fascinating Wikipedia page. For whatever reason, people in the East End of London have habit of “replacing a common word with a rhyming phrase of two or three words and then, in almost all cases, omitting the secondary rhyming word (which is thereafter implied), in a process called hemiteleia.” For example, the phrase “to blow a raspberry” to mean making a flatulent sound with your mouth and lips actually comes from the cockney rhyming slang for fart, “raspberry tart,” shortened to raspberry.
The Wikipedia suggests that “popping” is a cockney slang term for pawning, and weasel — from “weasel and stoat,” whatever that means — is cockney rhyming slang for “coat.” So it seems possible that “Pop Goes the Weasel” actually means to pawn your coat, which makes a lot a sense in the context of this verse, noted as early as 1856:
Up and down the City Road
In and out the Eagle
That’s the way the money goes
Pop! goes the weasel.
The Eagle was the name of a pub on City Road. So it seems eminently likely that the song we all sung as children is based on one about pawning your coat for booze. Better that than the black plague, I suppose.