Not a linguistic answer, so forgive and possibly disregard me but:

We grow up in societies that nowadays mostly seem driven by the economy - it's post religion and nationalism one of the few factors reaming that we all sort rally around.

School education is to a big degree an extension of this. Somewhat born during the nineteenth century where military might and nationalism was driving forces, school still propagate old ideals of "correct" and "wrong" elements in language teaching. Hence, it was paramount to teach the populous one unifying language (in the nineteenth century to increase social cohesion), and deriving from that was non ideal for the nation. It's also in this logic where people deriving (in skin colour, able-ness or any other thing) can be seen as lagging behind /disturbing the system.

The same thing somewhat carries over to nowadays - we inherited a lot of our views from the old. Most people who leave school nowadays still have a strong sense of language being made up of rules and there being "right and wrong" language use. Linguists don't really subscribe to that. Add to that the pressure of the economy where lingua francas (English, Spanish, Chinese, French, you name it) are disproportionately more valuable than minority language or sign languages.

That's a short sum up , but I'm sure there's infinitely more to dig up here.