The way I see it is that Glenn Beck is popular because he promotes fear. Fear is a very powerful motivator in America. It is what drives politics in general. Fear of the immigrant, fear of the differently-coloured, fear of government, fear of difference. If you listen to these guys (and I mean Rush Limbaugh, Sean Hannity, Michael Savage, et al), you'll realize all they're peddling is fear. No real solutions to our very real problems.
For the people traditionally in power (white, Christian men), he provides the hysteria to the soundtrack of their power slowly eroding. He is not popular with other ethnicities. He is not popular with Muslims or Buddhists or atheists. He is not popular with those but to whom he represents, the traditional power base of the US.