jQuery is a lot smaller and less nebulous than its competitors (looking at you,~~React~~ literally every JavaScript framework).

Jquery was what was popular when i learned js. I'm kinda glad it was, honestly: jQuery is a little unique in that it doesn't have magic to it the way js frameworks do. Everything you can do in jQuery, you can do in vanilla JavaScript pretty easily. With, say, React, how is a newcomer supposed to understand how a series of React components become HTML?

So jQuery kept it "real" for me. Fewer abstractions between me and the HTML meant it was easier for me to connect the dots as a self taught developer.

As for how it's changed, it's more any how vanilla JavaScript has changed. A lot of the things that made jQuery so much easier and cleaner than vanilla are now baked in, like document.querySelector(), element.classList, createElement(), addEventListener()... It had Ajax methods that, before jQuery, were a tremendous pain in the ass.

jQuery was great, but, you basically had to use it with something like PHP, because it had no back end. So when angular came out (and a few others that aren't around anymore and I've forgotten), it allowed you to replace both PHP and jQuery, and developers rejoiced.

Why did they rejoice? I'm not actually sure there was reason to, objectively speaking. As developers, we like new tech, especially if that new tech requires us to think about code differently, even if, in retrospect, it's a hard argument to make to say that, if we had just stuck with PHP and jQuery we would be somehow worse off than we are with React.

Of course, in tech, when a new system changes how we think, sometimes (not as often as we'd like) it helps us reconsider problems and find much more elegant solutions. So, would we have all the innovations we have today if all these js frameworks has never existed? Obviously we can't really answer that -- but it's a toke of copium for when we get nostalgic for the PHP/jQuery days.

(Also, for you newer people reading this, you should probably be aware that the PHP/jQuery mini-stack is still very quietly used. You'll definitely see it, especially in php-baaed COTS.)