Independent voters represent the majority in this country
You know nothing.
Scott Ramirez

Sorry, Scott, but your premise has been long since debunked. In fact, independents may actually be more partisan than voters who identify with a party.

When you consider the ongoing polarization of the electorate (the implication being that there are two major political poles), it should be obvious that the ongoing struggle to define America is being fought between the left-wing represented by the Democrats and the right-wing represented by the Republicans. Unfortunately for lefties, the right much more reliably falls in line behind their candidates. Mobilizing the left-wing is like herding cats (pardon the cliché), hence the frustration.