There are at least two ways in which this is true.
First, a two-party system tends to reproduce itself in the minds of the people. As we grow up, we are socialized to know that there are Republicans and there are Democrats. We are often socialized to know which we are. We are not socialized to feel a connection to any other party because there is no other major party. Therefore, we grow up identifying with a party. This tends to reduce our desire to have any new parties.
Second, and perhaps more importantly, our system reproduces itself because it is in the interests of the parties to have it so. It does not help either the Democrats or the Republicans if it is easy for third parties to come to power. Therefore, the parties are going to be reluctant to put forward any sorts of proposals (like proportional representation) that would encourage other political parties to form and which would help them to become significant.
In these ways, we can say that we have a two-party system because that is the system we have always had.