What is the origin of this phrase? Why “broke” and not “broken”?
Wild Arsed Guess: It’s meant to portray extreme conservatism and fear of change and modernity, and sounding like a hillbilly captures that nicely.
Now garn! Git orf mah laaand!
It seems to be attributed to Bert Lance
Lance was an advisor to Jimmy Carter.
“Oft to better we mar what’s well.”
Not really. “Broke” for “broken” in American English is just kind of folksy. I think it’s meant to portray a kind of common-sense, folk-wisdom kind of attitude.
Why “ain’t” and not “isn’t”? Same reason, I’d assume…a dialectal affect to make it sound like simple-and-to-the-point “country.”
[size=1]…said the West Virginian-born Californian.
I don’t think the coiners of the phrase had their MLA handbooks handy at the time.
Lessee now, young 'un. ‘Ain’t’ and ‘broke’ have a nice ring to 'em, that’s for sure.
Hard to keep up with you city folk. Fancy some lemonade? Ah squeezed it m’self.
Sorry, but I worked for an old Navy Chief, in the early 60’s, who used the phrase all the time.
yuh mean mahself m’boy 
Do you happen to know where he was born, raised, and from?
Balance, and rhythm, that’s why. If the coiner of the phrase had said, “If it isn’t broken, don’t fix it,” the rhythm would be lumpy, and it wouldn’t have that staccato tang of “If it ain’t broke, don’t fix it.” That’s my take on it.
I’m sure he was from the south, but I don’t know what part.
If it is not broken, Do not repair it.
Sorry, just aint got the same ring.
I don’t have any citations, but I’m sure the phrase pre-dates Bert Lance’s use in 1977. I think the phrase originally referred to things that were physically unbroken, not metaphorically unbroken. “If it ain’t broke, don’t fix it” was standard advice to a tinkerer - for example, a backyard mechanic who decides to rebuild a carburetor just because it’s been six months since he did it the last time.
As for why the phrase uses “broke” and “ain’t” instead of “broken” and “isn’t” - these words make the phrase sound homespun. They invoke the image of the wise rube - the person without formal education who is nonetheless full of common sense.
It’s so obvious that even those with imperfect English know it, and those with better English (seemingly more intelligent) sometimes ignore it and get poor results.
One WAG as to a possible origin. When Henry Ford put out his Model T he knew that many of them would wind up in the hands of inveterate tinkerers who would want to fiddle with this new gadget. So the owner’s instruction manual started off with:
IF THE CAR IS RUNNING, LEAVE IT ALONE.
I first heard it in an auto repair shop in the early 70’s, and have no doubt it arose way before then. Likewise the favorite question of my boss at said shop (to me, the only college graduate in the place) - “If you’re so smart, how come you’re not rich?”
Bert Lance may have introduced it to a bunch of white-collar folks and intellectuals who hadn’t run across it before, but I don’t see the cite above claiming he originated it. Which is good, because he didn’t.
I’ve always understood it differently - along the lines of Shakespeare’s “rather bear those ills we have than fly to others we know not of”.
And I agree that it certainly pre-dates 1977.
I can see where you’re coming from, but in all the use I’ve heard it’s a much more practical thing: don’t muck around with something that’s performing perfectly well - it’s almost always a waste of effort, and you’re much more likely to make it worse than to improve it.