Due to following advantages:

► Easy to use and learn.

► Easily expandable.

► Cross-browser support (IE 6.0+, FF 1.5+, Safari 2.0+, Opera 9.0+)

► Easy to use for DOM manipulation and traversal.

► Large pool of built in methods.

► AJAX Capabilities.

► Methods for changing or applying CSS, creating animations.

► Event detection and handling.

► Tons of plug-ins for all kind of needs.