Just guessing here; I think this is because you're pad sink types are
"sometimes" pads instead of "always" pads. This makes sense for a demuxer,
but you can't connect to them as regular static pads because they don't
always exist. To connect sometimes pads, you typically respond to a
pad-added signal. See docs for more info, or try experimenting with existing
muxers (i.e. matroskamux) before testing your element.