This has to do with the rise of a huge class of educated professionals, they fraying of local communities, and even with the movement of women into the workforce. (Women used to supply much of the drive to grass roots volunteer organizations.) It also has to do with the fact that many traditional grass roots associations were organized along sexual or religious lines that many now see as discriminatory.To this should be added the not small effect of campaign “finance” reform and other election laws. If it takes a professional to stay out of jail while being involved in politics, naturally politically active groups will tend to be controlled and run by only the hard core political junkies. An average group of citizens who tried to get involved in an election would almost certainly end up broke from fines or in jail. It seems to me that this might be an influence as well.
One wonders if this isn’t part of the reasons the Left is supportive of election law. I frequently get the idea that the Left, despite its “activist citizen” rhetoric, considers politics too important to leave to the unenlightened masses. The state to which political parties are evolving is completely consistent with this world view - a mass of contributors and bodies directed by a small, elite vanguard of the enlightened, just like a government should be. That doesn’t seem like a coincidence to me.