YUI was released the same year as jQuery. Care to qualify that a bit?
Edit: Had my dates wrong.
Edit: Had my dates wrong.
To top it off, the documentation was top notch.
Serious? That's what drove me away. It might have been top notch academically but for real life use I found it painful to get answers and simple code problems fixed.
The top hit on Google for "make an ajax request with (YUI|jquery)" the documentation page is the first hit.
YUI's page is here, the line of code to make the request is well below the fold. http://developer.yahoo.com/yui/connection/
JQuery's page has it listed immediately. In fact, 2 variants above the fold. http://api.jquery.com/jQuery.ajax/
I'm not saying one is right and another is wrong. I'm just saying jQuery's gets more adoption. It's easier and people tend to like that.
I don't, for example, understand why I have to scroll past the YUI dependency configurator to get to the syntax I'm looking for.
I just know that when I did try it (2009) I felt the same as I do today.
1. Yahoo hosted scripts, including roll-ups
2. UI components and styles
3. The best documented library, both outside and inside thde code, hands down
4. More feature complete (ex. one I can recall off the top of my head, form handling of upload inputs didn't exist in jquery, and extending jquery internals was/is a nightmare in comparison)
5. The best documentation, hands down
6. Yahoo hosted scripts, including roll-ups
7. The best documentation