ffmpeg is a fairly complex project which deals with a number of esoteric topics (audio/video file formats, compression, processing, etc). The number of open-source developers who are qualified to work on it, and who have the time and inclination to do so, is fairly limited; I suspect that there simply isn't a large enough population of those developers to support multiple actively developed projects in this space, and certainly not two as similar as ffmpeg/libav were.