The reason Americans Elect was unable to generate much popular support is simple: Not many moderates believe both that President Obama is too liberal and Mitt Romney is too conservative. Further, not many moderates feel there is no place for moderates in either the Republican or Democratic parties.
Given the narrative weight behind the idea that polarization in American politics is alienating a vast swath of the center, these claims may seem counterintuitive. However, a look at the raw data from two recent Daily Kos/SEIU State of the Nation polls provides strong supporting evidence.
First, in the Daily Kos/SEIU poll conducted from April 26-29, only 35 out of the 1,000 registered voters surveyed indicated both that President Obama is too liberal and Mitt Romney is too conservative (see questions 5 and 6, a .zip file with the raw data can be found at the bottom of the page linked). Only 18 of those 35 respondents self-identified as moderate, or about two percent of all registered voters surveyed.
Second, in the Daily Kos/SEIU poll conducted from May 10-13, only 92 of the 1,000 registered voters surveyed thought there is no place for moderates in either the Democratic or Republican parties these days (see questions 12 and 13, a .zip file with the raw data can be found at the bottom of the page linked). What's more, only 14 of those 92 self-identified as moderates.
In looking to find a viable, centrist, third-party presidential candidate for moderates who feel left out by both the Democratic and Republican parties, Americans Elect was drawing on roughly two percent of all registered voters for its base of support. As such, it is unsurprising Americans Elect was unable to generate sufficient participation to produce a nominee. While the idea that the two major parties have both abandoned the center may be popular in the opinion sections of several prominent news organizations, that idea does not have much traction with the American people.