In addition, YUI3 can dynamically load resources, which can be very beneficial if you have a large collection of widgets and only want to load the code for them when it's necessary.
I love both libraries, there are trade offs with each, which is why evaluating them for your project is a wise decision, if only to understand the strengths and weaknesses.
The overwhelming reason is we need guaranteed support for all the advanced features we want to build on. JQuery is a set of element actions/seclectors, but lacks plug-ins/widgets with the core library. JQuery has active developers all over the web building plug-ins that are amazing. But if we incorporate with the plug-ins and find a bug, we have to hope the plug-in project is still active and will receive support.
With YUI, we can use their supported widgets, file bugs and know we will receive attention from the YUI developers.
Also, note the author's choice of code is not always the simplest and could be better written.
Ask HN: If you actually prefer working with YUI3, tell me why?