There's been some interesting analysis of the perceived "polarization" of the United States. It turns out that we, the American people, are not more polarized than we have been in decades past. But the political parties are.
It used to be that both parties had a spectrum, from liberal to conservative. The South tended to be very conservative, but they were mostly Democrats, because after the Civil War, no white Southerner wanted any part of the "party of Lincoln." Similarly, the north held many liberal Republicans. Socially liberal rich people, the so-called "Rockefellar Republicans."
But now, both parties are more polarized. The "boll weevils" (conservative white Southern Democrats) have mostly switched to the GOP. Meanwhile, black Southern Democrats tend to be much more liberal than their white predecessors, pulling their party left. And elsewhere in the country, the Rockefellar Republicans are being replaced with a more rightwing crowd.
The result is that "Independent" is the fastest-growing segment of voters. In some states, there are more independents than Republicans or Democrats. It used to be that almost everyone who voted was a member of a political party. No more.
It does seem like that leaves room for a third, moderate party to rise up in the middle. We almost got that with Ross Perot's "People's Party." Unfortunately, he turned out to be a megalomaniac nutcase.
Anyway, at least one reason fewer of us are members of a political party is the increased complexity of our society. The population is larger. No longer can party operatives knock on doors and talk to everyone individually. (Even if they try, we're not home, or we don't answer the door or the phone, being too busy.) The parties have switched from offering rides to the polls to blanketing the net and the airwaves with ads. Only the hardcore are left, in both parties. Mainstream America has been left out.