Trusted contributors could easily be bought off. Someone who was sick and tired of StackOverflow after putting way too much time into it, who was offered the right amount of money? Hey, you know, it might be worth it to make a few bucks instead of a few more meaningless internet points.
If you just make the threshold so high that only the very top contributors are nofollowed, then only that small cabal controls where SO's considerable linkjuice goes. If you broaden your criteria, there are more and more potential corruptible people; and it becomes easier to generate a little quick and easy reputation with a sockpuppet that you can spam with a few times before you're caught and move on to another sockpuppet.