A bit of the trick and appeal to animated gif is no sound (at all) and (typically) low cruft: it's the 'money shot', looped. no padding, no channel begging, optimized for download speed, no buffering, no pop-ups, no ads, no captions, etc.

I wouldn't mistake its prevalence as evidence of any failing of <video>. When people -want- video, they know how to post, embed or share it. While geeks care about plug-in-free video, the people sharing animated gifs do not.