It won because it has a great API and excellent documentation. jQuery is pretty much the only library or framework made 10+ years ago I can use and still feel

It won because it has a great API and excellent documentation. jQuery is pretty much the only library or framework made 10+ years ago I can use and still feel like "this is actually well-designed". Frameworks generally obviate the need for it, but if a framework isn't an option and if I'm going to be writing more than 100 lines of code (otherwise I'd suck it up and use plain JS), I always still use jQuery.