To maintain the illusion of choice and encourage participation by voters and keep donations high, its vital to keep the competition going. Sure there are constituencies who benefit from the largess of government that support each party, but that maintains the huge cash flow that's generated from the election cycle. The real truth is, as Ron Paul has said, there is only one party, the big government party. No matter what party manages to get in power, it spends, borrows, fights wars, restricts freedoms, taxes and most importantly it grows government.
The good news is, people are figuring it out, that's why the voter participation was so low this year, the illusion of choice is fading.
The bad news is, they will not allow the growth to stop.