Why does Henry Higgins teach Eliza Doolittle to speak like a posh lady, instead of her teaching him to speak like a Cockney flowerseller?
What we think of as “good” English is the English historically spoken by people with the most power. The bumper crop of grammar texts and usage guides that started proliferating in the mid-18th century were part of an attempt by the growing middle class to access economic opportunities that were only available to people who spoke like Henry Higgins. At first, these were primarily a guide to speaking like the upper classes, although, over the years, various arbitrary preferences have found their way in and became crystallized as dogma, so much so that, to quote the linguist Stan Carey, “the aim of these non-rules is to maintain anachronistic shibboleths that allow an in-group to congratulate itself on knowing them.”
Can it be a rational decision for the Elizas of the world to modify their idiolect in search of more opportunity? Of course. But at a societal level, it’s deeply suspicious that Henry gets to grow up speaking in a way that automatically makes him a better job candidate, while Eliza will have to learn a different dialect than her friends and family if she wants a chance at the same jobs.
We don’t pick where and how we grow up, and we know that where and how you grow up influences your idiolect, so why is it acceptable to penalize people for something no one has any control over? The answer is simple if your goal is to keep power and economic opportunity in the hands of those who have always had it. We like to think we’re more enlightened and less bigoted than our ancestors, but as long as we believe that some idiolects are right and some are wrong, we’re not making much progress. “Standard English” is a loose assortment of idiolects like any other dialect, and valuing one over the other is a social construct that has nothing to do with linguistic merit.
In which I explain idiolects and Hulk-smash prescriptivists.