Perhaps it's no surprise, given the fact that this unfortunate queen of France was born in 1755 -- an age of high infant and childhood mortality rates. And there are numerous other ways in which one's matrilineal descent can expire -- e.g. deaths in young adulthood unmarried (remember that poor medicine has been the rule throughout history), perhaps death from complications in attempting to give birth to one's first child after marriage, women who took up religious life or for some other reason never married, childless marriages (for practical reasons I count any union such, if it failed to produce a single child reaching adulthood), marriages that produced only sons and no daughter.
So of her ten sisters, I believe only a few survived to adulthood, married, and produced daughters. But then, the same story would repeat itself in the next generation, regarding those nieces through her sisters (e.g. deaths in infancy or childhood, etc).
Or perhaps I'm mistaken, and the maternal collateral descent from Marie Antoinette is extensive -- meaning that there are other surviving lines. Does anybody know of additional ones (whether from Maria Carolina or another sister, perhaps from another niece)? It has been said that the late Queen Anne of Romania represented such a line: since she herself was the mother of five daughters, the said line is still alive and flourishing.