It’s tiny. no overhead. It also has a well supported community. Which means plugins galore. And it based off of CSS syntax. So there’s very little learning curve.
And the best feature of all… it’s cross browser compliant!!
With MooTools and Prototype, you have to be careful not to use the same variable name as their API. But not with jQuery. It is completely isolated in it’s scope and won’t affect any global namespace.
Yahoo UI and Dojo, well those are big boys. And i mean HUGE files. I prefer small footprint over bulky features. In fact, I’ll stab my eye out before I use Dojo… but I’ll digress.
For more info : http://htmltree.com/the-beauty-of-jquery/