When I first was conducting as guest conductor in Europe 25 years ago, I would propose doing American pieces and grudgingly it would be accepted from time to time.
The real deep text of music and the whole reason that it has continued with the profundity and urgency that it has for over a thousand years, has to do with what the notes say, what the notes witness, different experiences of hope or doubt that people are able to distill and encode and pass on in this way.
But a large symphony orchestra basically is a repertory company and it has a very enormous repertoire and it is important for the performers to be able to know how to shift focus so that they instantly become part of the sound world that a particular repertoire demands.
The first year I started in San Francisco, there was an American work on every program and there's been a lot of music by living composers and gradually that was part of the process of getting the audience really to trust me.