"Mary was reported to be always making remarks like, 'Why don't they eat cake?'"
This is derived from the phrase 'Let them eat cake' supposedly uttered by Marie Antoinette (1755-1793), wife of King Louis XVI of France, in response to the complaints of the French peasantry about the lack of bread.
They are, in fact, a mistranslation of the words 'Qu'ils mangent de la brioche' ('Let them eat brioche*) attributed by Jean-Jacques Rousseau in his Confessions to 'a great princess'.
It has been suggested that the princess in question was not Marie Antoinette but Marie-Thérèse, wife of King Louis XIV.
*a light, soft-textured bread made with eggs, butter, flour and yeast.