I'm going to have to challenge your assumptions on this one, as you seem to have fallen victim to the Stepmother Faction's propaganda. We have multiple early records of Ella's life, but it is not until the late 19th century that her noble, or at least wealthy, orgins are removed, suggesting that certain parties with a vested interest in claiming the throne may have worked in concert to cast doubts on the princess and thus delegitimizing the Charming dynasty.

From our earliest records, found in Italy and dated from 1634, the woman who would become best known as Cinderella was in fact a Princess, betrothed to Charming through the usual methods, whose governess (and later progenitor of the Stepmother faction of pretenders to the throne) attempted to trick the prince into marrying one of her many daughters in lieu of his intended, thus securing her claim.

60 years later, the noted early French historian Charles Perrault published a new account of the Princess's life in which he claimed she was the daughter of a wealthy man, but not a prince. Here we can see the first signs of Stepmother influence on the record, though Perrault can hardly be faulted for his conclusions--the science needed to conclusively prove "Ella's diary" a forgery simply weren't available for another two hundred years. Other details claimed by Perrault, however, are harder to explain through the historical record, notably his insistence that Cinderella was, in fact, assisted by a "fairy godmother." Some later scholars, particularly those of the “Once Upon A Time” school of thought, have attempted to show that this “fairy godmother” was, in fact, an agent of the Stepmother. In this school, the fairy godmother was indeed sent to Cinderella to make her story of escape and redemption implausible. However, it should be said that “fairy” could also have originated from a mistranslation of the older Italian word used to describe a certain class of wise woman who chose to live apart from society. This is the view I subscribe to, personally: According to this narrative, on the night of the ball, Cinderella left her home and fled to the woods, where she found her godmother, a wise woman, and begged for help in getting to her fiancé and warning him of the Stepmother’s plot.

By the 19th century, historians and linguists Jakob and Wilhelm Grimm also attempted to attack Cinderella’s parentage, claiming that she was the daughter of the wealthy nobleman’s first wife, explaining why that gentlemen had little interest in protecting the child against his second wife’s depredations. It’s not until the early 20th century, when Walt Disney approached the topic in his pop history documentary, simply called “Cinderella,” that she was first considered to indeed be the peasant she appeared.

So what we can see is a long-term campaign to delegitimize the Charming dynasty by claiming that the marriage between Charming and Ella was invalid under the laws of the time—marrying a commoner simply would not be tolerated, even in this day and time, and so Ella’s history must be reconceptualised to make her the peasant she seemed to be, rather than the princess she was by birth.

Edit: Forgot the second half of a sentence.