"The two party system forces people who advocate for issue X to also have to advocate for Y and Z, when they may really only care about X. Another factor; the decay of respect of and audiences for traiditional mass media, and the rise of personal "bubble" media such as social media has also forced mass communications to be more personal if one wants to reach people, and various political forces are adapting to the new landscape."
I'm not sure if population density has any effect on political discussions more than discussions in general.