Why have the Democratic and Republican parties been so durable so as to maintain exsistence since the Civil War?

There can be many ways to go about answering this.  I
would take the economic materialist point of view.  For the most part, both parties have
not represented a true departure from the capitalist practices that have helped to
define America in the last 150 years.  I think that this is part of the reason why they
have endured.  In the end, while America strives for different expressions of the
political notion of the good, industrialization and commerce have helped to make its
economic articulation a capitalist and free enterprise one.  The Democratic and
Republican parties have, for the most part, represented an embrace of these ends.  There
have been moments where this has not been as true, but for the most part, one can say
that both parties have done their part to support free enterprise and capitalism as the
primary means of economic exploration of the good in the nation.  This has been
certainly the case over the last two decades, and has helped to explain why the
Democratic and Republican parties, despite political differences, end up sitting at the
same economic table of profit and wealth.