I'm not sure why people are so shocked by this - I've been saying it the whole time.
Okafor is a one dimension player - his only talent is his back to the basket game. NBA teams rarely ever place high value on guys who only can only do one thing at a high level - it's been made obvious all through history. It makes perfect sense - if I were an NBA GM, I wouldn't place a high value on one dimensional players either.
That's why a guy like Butler (who can pass, handle the ball, score, defend, rebound, get to the line) is so much more valuable then a guy like Bradley (who can only really shoot and defend).
There's a simple reason for this - teams want to know they can get constant production out of you. Nobody is going to grab 15 rebounds every night. Nobody is going to score 30 points every night. Nobody is going to get 10 assists every night or hold their opponent to 25% shooting every night.
Teams want to know that if your one great skill happens to be off one night, you can do other things to help your team win....but if Okafor isn't scoring in the paint, he's a major liability in every other aspect of the game. His ability do dominate / produce is entirely dependent on favorable match ups on any given night.
Noel has the defensive versatility to be able to defend quick bigs, long bigs, perimeter oriented bigs, and he can switch onto guards and forwards because of his foot speed. That makes him a valuable player. But even if (by some chance) the defensive matchup doesn't go his way, he can still impact the game with his rebounding - which isn't great, but is at least on par with what you expect from a center. Plus you never have to worry about his conditioning and he doesn't seem to have any character concerns that might be problematic.
It's blatantly obvious that if the Sixers want to trade Okafor rather than Noel, then they believe Noel is the more valuable player to them. Other GM's think the same thing, for all of the above reasons.