why was the "we only use 10% of our brain" myth created?

Same reason every other myth was created: Someone wanted to sound smarter than everyone else.

Think about it... why else would anyone make stuff up? If you immerse a steak in Coca-Cola and leave it overnight, the soda will dissolve it. Did you know KFC changed their name to KFC because it's not really chicken so they can't legally call it that? Guess what, I know that if three crows are outside your window on a Tuesday, it means someone is gonna die. The world is secretly ruled by giant snakes from outer space, and the government knows all about them. And you'll go blind if you don't stop doing that. It's all true, just ask my cousin's best friend's classmate's older sister if you don't believe me!

See how it works? I just listed off half a dozen (or whatever number) myths just off the top of my head, and you paid attention. That's better than you not paying attention, because I'm an attention hound. If I wasn't addicted to attention, I wouldn't have said that stuff.

That's why.

A lot of people use very little of their intelligence.
There are still many teachers - and priests! - who just want children to memorise boring crap and never think outside the box.

So the claim that "we only use 10% of our brain" is not literally true; but it still communicates something that is true.

One possible origin is the reserve energy theories by Harvard psychologists William James and Boris Sidis in the 1890s who tested the theory in the accelerated raising of child prodigy William Sidis; thereafter, James told audiences that people only meet a fraction of their full mental potential, which is a plausible claim. In 1936, American writer Lowell Thomas summarized this idea (in a foreword to Dale Carnegie's How to Win Friends and Influence People) by adding a falsely precise percentage: "Professor William James of Harvard used to say that the average man develops only ten percent of his latent mental ability".[6] This book was not the first to use the 10 percent figure, which was already circulating within the self-help movement before then; for example, the book Mind Myths: Exploring Popular Assumptions About the Mind and Brain includes a chapter on the ten percent myth which shows a self-help advertisement from the 1929 World Almanac containing the line "There is NO LIMIT to what the human brain can accomplish.

Because of stupid uninformed people.

They don't know what brain does and don't want to admit spirit so they want to push atheism on everyone.