The biggest reason for this phenomenon is that the raters who are most prone to like a course by a particular designer, are the ones who go in the first two years and set the high bar for it. Then the guys who don't like, say, Jim Engh's work think, well, maybe this course is different than his others, so they go to see it, and it's not different, and their votes make it fall back to earth.
As Jeff says, too, the private courses get a lot of attention when they're brand new and the developer is spending marketing dollars and the course gets ranked as Best New ... but if it's private, that's the last of that it will ever get, because you are never going to see articles about private courses in the mags. So from there, there is nowhere to go but down. The same phenomenon explains why tournament courses [which get a ton of attention when they host an event] generally have more staying power, as long as they keep hosting an event.