With JS, the answer is because Netscape adopted it, and it had market share. If you wanted to code which ran in web browsers, you needed to use it. Netscape, the company, no longer exists, so I wouldn't call it all too successful an outcome. From there, momentum.

Early history is in '95, Netscape hired Brendan Eich to embed Lisp in Netscape, while simultaneously pursuing a partnership with Sun to embed Java. Sometime that year, Lisp changed into a new language, which was created and shipped in beta, by September 1995. We're talking a few months.

If we had designed a sane framework upfront -- even 6 more months design time -- the web would likely be a decade or two ahead right now.

Most of the early "improvements" to JavaScript were stop-gaps created under similar cultures, which would need to be replaced again and again. It's not that we didn't know how to make a decent programming language in 1995; it's that we didn't.

Today, I do about half of my coding in JavaScript since it runs in the browser, and therefore on all platforms. It has nothing to do with the painfully bad language design.