Because it is likely that a poster on an imageboard made the video themsevles, rather than reposting in an environment that may only permit simple image links,

Because it is likely that a poster on an imageboard made the video themsevles, rather than reposting in an environment that may only permit simple image links, it encourages them to save it as webm instead of distilling to a GIF (not easy to do properly, very bandwidth intensive on view), which allows the videos to be longer and higher quality while file size caps can be lowered. Mobile users are also thankful for this trend.