Earlier quoted context omitted.
jQuery filled a need. It's as simple as that. Its success has nothing to do with looking in the other direction or not redefining Javascript: it solved a problem that a lot of Javascript developers were struggling with. Now what's really fascinating to me now is that while one can safely say jQuery created a revolution in the Javascript world when it came out, it is today widely looked as a smell and avoided as much…
That need was filled by Prototype.js before jQuery. I remember the times when Rails defaulted to Prototype (author was a member of Rails core team, iirc). I also remember when we still had to choose between the two for the new project and I was unhappy if the choice was Prototype. So they both solved the problem, but jQuery's solution was more focused and way more elegant.
jquery was just easy dom selection and manipulation, easy ajax, and easy events... you could drop it into any project and it didnt really proscribe anything else...