Here's another, less obvious reason why the Right wins.
Angry conservatives take over the GOP and push it rightward.
Angry progressives flee the Dems and join a third party or an advocacy group.
In the conservative movement, disaffected groups, such as religious conservatives and Tea Partiers, mobilize their anger and take over the GOP (with substantial help from rich benefactors such as the Koch brothers).
On the Left, disaffected progressives often give up on the "hopelessly compromised" Democratic Party and vote for a third-party candidate, such as Nader, or join an advocacy group like MoveOn, Sierra Club, NARAL, PDA, DFA, or hundreds of similar groups.
GOP and Dems by Don Smith (from Art Explosion)
I have progressive friends further to the left of me who often say things like, "Oh, you can't trust the Democrats. They'll always betray you. They'll use progressive rhetoric to get your votes, but in the end they'll escalate the wars and vote for Wall Street, Big Insurance, and the other corporate interests."
Another, prosaic reason why progressives give up on the Democratic Party is that taking over a party organization is difficult and unpleasant. Blue Doggish Democrats fight back. Even the progressive activists are often difficult to get along with. Party meetings are long and boring. It's so much more fun to start your own party or lead your own chapter of an advocacy group! But that's not very smart. There are already too many lefty groups.
So, too often progressives flee the Democratic Party, causing it to become more "centrist" and robbing progressives of effective power.
Progressives don't seem to appreciate the fact that in our non-parliamentary form of government, third parties have little constructive power. They also don't seem to realize the importance of local Democratic Party organizations.
Partly the fault lies with the Democratic Party, which hasn't defended a populist, progressive agenda and hasn't welcomed progressives into the fold. There is mutual anger and distrust between centrists such as President Obama and progressives, who have been feeling very unloved and unheard. Progressives (e.g., single-payer advocates) have been unwelcome at the table in many Democratic organizations. Obama seems to go out of his way to alienate progressives, who are nearly in open rebellion.
I see this mutual hostility locally too, in legislative district and county organizations. In party committees, the upstart progressives often leave in exasperation due to the opposition. This harms both them and the Democratic Party. Many progressives attend meetings of advocacy groups but shun the local Democratic Party meetings.
Centrist Democrats such as Obama try to blame the Republicans for the Dems' inability to pass progressive legislation. While certainly the Republicans are terrible, the evidence is clear that Obama has compromised early and often; has continued many of the policies of the Bush Administration; has surrounded himself with Bush holdovers; and has failed to hold Republicans accountable for the mess we find ourselves in, thereby hiding from the American people the truth about the recent past. He got nothing in return for his many compromises except a big loss in the midterm elections.
Likewise, it's unreasonable to blame Obama's failure to lead on his alleged inability to communicate or on his naivete about bipartisanship. As Glen Ford says, "Of all people, Barack Obama doesn't need communications lessons." Obama is a smart man. He knows what he's doing.
So, the Democratic Party has driven itself into a rut, by failing to market progressive ideals and by allowing conservatives to control the narrative. Perhaps Democrats feared losing corporate funding and feared being called Socialists by conservatives. So now too many Americans think that government is the problem and that dismantling the New Deal and cutting taxes for the rich are the way to prosperity.
But after the recent election losses, Democrats are finally coming around, I think, and realizing that they need to be more progressive and show more fighting spirit.