I think the main issues were accessibility, IE support without a canvas stand-in which could be very slow, and accessibility. Also, general rendering is pretty speedy on all browsers, whereas canvas rendering was pretty variable until recently (and still is for fast redraws).

Also makes theming changes quite a lot easier using css.

(http://groups.google.com/group/ace-internals/browse_thread/t..., http://groups.google.com/group/ace-internals/browse_thread/t...)