I never understood that. You'd think that reddit users are savvy enough to adopt html5 video. RES could just as readily inline video as they do animated GIFs, and we would all save a hell of a lot of bandwidth.
The advantages of animated GIFs made sense pre-video tag.
It's probably more of a perception thing than anything else. An animated GIF feels like a lot less overhead than a full on video, even if the file itself is smaller as a video.
Also user forums like reddit are a lot more likely to permit images in posts than a video tag, so you get the added portability of being able to post it almost everywhere.
The advantages of animated GIFs made sense pre-video tag.