Alex Ross has a fine article posted at TNY about the 20th c. symphony, with a follow-up blog post. Between them, they are an implicit criticism of orchestras' failure to program symphonies outside the usual 18th and 19th century suspects. He links to Bob Shingleton's On an Overgrown Path blog, and I definitely encourage reading those links.
Alex writes about a number of composers, mentioning those whose symphonies have come closest to entering the standard rep. SFS programs a couple of Shostakovich's symphonies annually, and I'd say he's awfully close to standard rep at this point.
For me, the most bizarre omission from orchestral programming is Ralph Vaughan Williams's symphonies. The composer is represented primarily by the Tallis Variations, The Lark Ascending, and other comparatively lightweight works, while his symphonies largely go unplayed. They are great works, serious and more interesting than the RVW you usually hear played in the US. I've seen the Fourth twice, conducted weakly by Botstein and wonderfully by Yan Pascal Tortelier, and the London, in a terrific performance by Osmo Vanska. I'm particularly astonished that SFS, with its magnificent Symphony Chorus, has not yet performed A Sea Symphony, which sets poems, or parts of poems, from Walt Whitman's Leaves of Grass.
These symphonies also pass current marketing-driven requirements for "accessibility." They are conventionally beautiful, even when bleak, and are works that a conservative audience is likely to find acceptable.
Elsewhere in discussions of canonical repertory, Speight Jenkins, former general director of Seattle Opera, published a truly foolish article asserting that the big problem in opera today is that singers don't emote enough. Okay, maybe we are attending different operas; I don't see this, and moreover what I do see is a lot of superb ensemble work, which, in a theatrically and dramatically sophisticated world, should not be overlooked or denigrated.
He goes on to make the absurd claim that Maria Callas and Leonie Rysanek couldn't get work today because of their combination of vocal instability and dramatic commitment. This is patent bullshit; I can name more than a few singers with erratic vocalism and great dramatic commitment, starting with Evelyn Herlitzius and proceeding to singers like Lauren Flanigan, Gwyneth Jones, and others.
Lastly, he says that he doesn't think the operatic canon is limited. Well, I call bullshit on him again. Here are useful statistics about most-performed operas and composers from Operabase.com, that immense storehouse of performance information. Read those tables and weep.
Needless to say, you can find similar statistics about what gets played at US symphony orchestras, via the League of American Orchestras. You'll cry when you look at their reports as well.