it was the first client side scripting language introduced by netscape in the browser war in '95 during the time it was dominant, submitted for standardization in '96 (and standardized a year later).
Microsoft just couldn't get a properly competing scripting language out fast/popular enough so they just adopted it as JScript.
Netscape made it to give their web browser an advantage, and then it stuck.
One of the things [Netscape] wanted to do was to put interactivity back into the browser, because we had lost interactivity when we went to the browser model. [...]
In order to do that, they hired this guy, Brendan Eich, who had been at Silicon Graphics. Brilliant guy. In his interview he said he wanted to write a Scheme interpreter, and they said ‘that’s great, that’s just what we want’. After they hired him they found out what Scheme was, and they said ‘no, no, no, you can’t do that. People won’t like that. Do something that looks more like Visual BASIC, or Java, something people like.’ [...]
Brendan took elements of all three of these languages [Java, Scheme, and Self], and a little bit of his own, and put them together into a new language that was called LiveScript. [...]
At Microsoft they’d been watching this with some alarm, particularly when folks at Netscape were saying that Netscape Navigator was going destroy Microsoft. Microsoft said ‘oh, we don’t want to be destroyed’. It turned out Netscape Navigator didn’t destroy Microsoft. In fact, the software that is going to destroy Microsoft is Windows Mobile.
The good news is that, for the most part, the bad parts can be avoided. And if you avoid the bad parts, and if you work just with what’s left over, the good parts, there’s actually a brilliant language there.
I think there were two major factors among many:
- Danish programmer Lars Bak changed the landscape of the internet when his team at Google engineered Chrome's V8 runtime.
- Steve Jobs.
There were a bunch of other scripting languages; Oracle created wild applets so you could use Java.
Remember when all you had to do was click here and wait while Flash was installed? Back then it was the only way cool things could happen, Flash's ActionScript became the true mark of high quality browser development with cutting edge UI/UX.
Suddenly the execution environment sped up dramatically with V8 and the browser wars had begun. As FireFox, Chrome, Safari, and many others fought it out, the execution time was under constant re-optimization.
One day, Steve Jobs woke up and decided to kill Flash.
So yea, probably Steve Jobs and Lars Bak were the two major reasons I would say.