According to nme.com, Michael Stipe doesn't want to talk about R.E.M.'s next album because he doesn't want to mislead people (and sabotage the record):
"He told Irish radio station TodayFM: 'I spoke early before the record was finished when journalists asked me what the record would sound like and I've learned my lesson on that - I'll never allow that anticipation to form into words again. Because I said it was going to be 'chaotic and political and very noisy' and it is, but it's a whisper rather than a shout. I'm not going to talk about it until it's done.'"
He feels that he got people excited about a certain kind of record, so they were disappointed when the band delivered another kind. There's definitely some validity to that, but I think no matter what he says, everyone wants another "Murmur," even though that's impossible. Perhaps one of the reasons R.E.M. has not become Advanced is because they're still worried about disappointing people who expect certain things out of them.