AJAX and the Web 2.0 hype around 2005 are responsible for the success of the last few years. There were also YUI, prototype.js, ExtJS before anyone used JQuery. But thanks to the popularity of JQuery around 2010 we got at least two JS functions in HTML5 browsers. And the browsers got more compatible with the HTML spec. Before you had to write for IE, Firefox and Webkit with fallbacks for various bugs. Since around 2013 vanilla Javascript is enough, and JQuery popularity is declining. There are React/Angular/Ember/etc nowadays but hopefully soon ShadowDOM will be supported by most HTML5 browsers.
Just to clarify: YUI and ExtJS both came out after jQuery, jQuery came out in 2006, and jQuery is actually more popular now than it was a year ago (or in 2010 - see the figures cited in the blog post).
I still use jQuery very heavily in projects not already built on frameworks. It's way easier for me to add a new module or function to a monolith like that by writing a little jQuery than trying to overhaul the whole project to use Angular/Ember/React/Vue/Mithril/whatever.
Thank god for jQuery for this exact reason. It's been a refreshing constant in the exhausting mess that is the JS framework world, and the fact is that most software is "monolithic."
jQuery is a dependency of Ember. Angular (at least 1.x) requires either jQuery or its own subset, jqLite. Cross browser incompatibility is still a thing and jQuery remains a very good tool for abstracting the differences.
