While watching tonight's finale of Dancing With The Stars, I noticed that the judges were being more lenient than usual. All three finalists received straight 10s and no sharp criticism. Even in the finals (the show before the finale), the judges scores were more compressed than usual.
Better Dancers Make it Farther. Or, each dancer improves enough as the season goes on that everyone is essentially the same skill level by the end. This is the simplest explanation. You start out with a wide distribution of talent and it compresses as the worst get eliminated and as the non-eliminated become better dancers. Naturally, this will lead to the judges scores rising over time and compressing.
That said, to my untrained eye, the judges were more lenient for a given quality of dancing. Tens are rare before the finals, but they're commonplace on the last week. Why be so lenient? I think it is mostly a combination of two factors.
Don't Rock the Boat. By the final week, judging cannot realistically provide feedback for future performances. As providing detailed nitpicks about the contestant's footwork isn't going to have the payoff for next week, the judges can relax and watch the show. Carrie Ann Inaba even said she forgot "she's supposed to be judging." No nitpicks ==> No demerits ==> Higher scores for everyone.
Let the Fans Decide. Consider some simple math. Half of the score is from the fan vote. The other half of the score is from the judges. This makes it sound like an even split, but it is not. If the judges give the same score to everyone, the only score that matters is the fan vote. More generally, the fan vote is more important because it varies more across contestants than the judges score does. By giving all of the contestants the same score (or even approximately so), the judges are effectively giving more discretion to the fans about who ultimately wins the dancing competition.
This last point shines light on how each season of DWTS proceeds. The judging math ensures that judges scores compress as the weeks roll on. As a result, the judges are never more important than on the first night and never less important than on the final night. What's the consequence? On Week 1, the show is about dancing more than it is about entertainment. On Week 10, the show is almost entirely about entertaining the television audience. If you like dancing entertainment, it is about the right formula for an entertaining program.