You’re ignoring the fact or maybe missing it, that the political landscape is actually changing. Voters are choosing third parties on both sides. Democratic defectors go Green and Republican defectors go Libertarian. The Libertarian party has more votes than the Green does because Republicans have been ignoring the values that voters vote for for much longer than the Democratic party. Now the Democratic party is going down the same path as the Republican party. The reason is mostly inertia but also to the fact that the political elite are tied through their umbilical cord to their traditional promises to donors and such.