Unless you have your own money to run a campaign, you need support from one of the major parties. They, along with their campaign donors, are only going to support candidates that share their view points. It's apparent that the Democratic and Republican party views have gotten extreme. This makes it difficult for anyone that's not a right-wing conservative or a left-wing liberal to win, when in fact nearly all the constituents they will go on to represent lie somewhere in the middle of these viewpoints.