I just get the impression, based upon David's reviews (and others), that Carnegie Hall - good as it is for most things (personally, I love the place) - must be too small do to do a bang-up job with Mahler 8. For one thing, you've got to haul in an electronic organ, which, to my ears, rarely sounds good. Then you have to extend the stage out, but there's still limited space for large choral forces. I'm sure Bernstein could have pulled it off, but he probably could have whipped up a great M8 in a high-school gymnasium.
Barry