I can only speak for myself, but IMO jQuery won because of their docs. I seem to remember having a hell of a time trying to get MooTools to work in the summer of '06, and finding docs to be of no help.
People underestimate just how important jQuery's documentation has been to its success. Even jQuery themselves seem to underestimate it, because the documentation for jQuery UI and jQuery Mobile really are not as good as the core docs (though still far better than the average). I probably would not have started using jQuery for my projects nearly as soon or as often had it not been for the excellent documentation.