People are good with dealing with a small number of simple things that can be stacked together. Throw in a human-readable data stream, and you're set to understand and use a stack of simple programs.
People are not good with dealing with a single monstrous object of unfathomable proportions, they will try to break it down in things they understand. If the thing is too complex, with too many inputs, too many outputs and too many states, this is a recipe for confusion. This is why overly complicated things always fail in face of simple things.
One could argue that FTP/SFTP was just as good as transferring bytes over network, but HTTP/1.0 won because it was simpler.
HTTP/2 was written to tickle the egos of its developers, following the principle - it is hard to write, it is hard to read. And its downfall is going to come from this problem.
