That is not true. YUI community is pretty vibrant and the developers go out of the way to help you.
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.