I recently heard that Sanders and Trump are both popular because they tap into the polar opposite yet equal viewpoint that the political system is broken and Sanders/Trump are non conformists of the political establishment who due to being outsiders can rule objectively without the bureaucratic corruption of most politicians and thus implement the effective changes to fix the broken system.
Now for Trump the system is broken via cultural problems, whereas for Sanders, it is economic problems. But they both maintain the image that they are outsiders who are uncorrupted by the government's bureaucracy and can thus fix it without falling into its trap.