Because this is a new way to provide binaries and we’re rounding off the rough edges. The problem with FFMPEG in particular is quite unfortunate, as it wasn’t initially caught by the continuous integration service that we use (AppVeyor) and the Windows installation on Travis is incomplete and broken for use with FFMPEG, thus unreliable.
The fix will go in the official FFMPEG.jl package in the next days. I made the fork to quickly test a possible fix that I found 