Its because the center failed to provide a path that people were satisfied with, hence there was movement away from the center into the fringes. It is entirely the center's fault that they are small. Maybe its just the end state of any sizable society.
The parties reshuffle themselves every few decades. Yes in the American system, it is inflexible to have more than two parties but the parties themselves are quite malleable and are following changing trends even though it may not appear to be so day to day. See the movement of Republicans towards far right despite the party leadership trying to stop it (they failed). The same seems to be happening on the Democratic side with them trending towards the far left although that might take a few more election cycles to bear out.
