Because the alternative would have been VBScript (if Microsoft had won at the time).
JS was a compromise. It had to be sent out the door quick, it needed to look sufficiently like Java to not upset Sun who were trying to establish Java as the universal platform at the time while not being feature complete enough to be perceived as a competitor rather than a supplement. And it had to be standardized ASAP to pre-empt Microsoft's Embrace Extend Extinguish strategy (which was well on its way with JScript). That's also why it's an ECMA standard rather than ISO despite Netscape not having been based in Switzerland - ECMA simply offered the shortest timeline to publishing a standard.
I think what's more amazing isn't just how we managed to build the bulk of user interfaces in JavaScript but how we Node.js managed to succeed with ECMAScript 3. Node.js was born into a world without strict mode and without even built-in support for JSON: https://codelucky.com/javascript-es5/ - and yeah, ECMAScript 3 was succeeded by ECMAScript 5 not 4 because it took vendors 10 years to agree on how the language should evolve in the 21st century - not only did we build the modern web on JavaScript, we built a lot of the modern web on the version of JavaScript as it was in 1999! Even AJAX wasn't standardized until 2006 when Web 2.0 was already in full swing.