I don't think the problem is conservative orchestras. There's been very little effort over the past 40 years to cultivate an audience for the music. And orchestras need more subsidies to afford to rehearse unfamiliar material that may not draw very large audiences. Of course they can't take chances playing Piston, Ives, and Schuman all season. Americans don't know any of this music. It's the audiences that are conservative and ignorant.
My own city's symphony orchestra folded a few years ago because they no longer received the corporate sponsorship from a large locally based company that went global like every other company, and they don't give a damn about local orchestras and musical culture. And an orchestra cannot survive off of ticket sales. They couldn't even sell enough tickets for a concert featuring a celebrity soloist, so the event was canceled. It's the same for jazz and progressive rock music. That's why jazz festivals are always booking pop acts. If it's all jazz, nobody shows up.