First off, I doubt the 60% figure because if the ratio was that high I suspect that you'd see a lot more used Wii Fit games show up on craigslist or ebay; beyond that, if such a high portion of people gave up so quickly I'm pretty sure word-of-mouth would have gotten around and Wii Fits sales would have suffered.
Now, I suspect there are quite a few people who give up within a month because Wii Fit is too hard or too easy for them, it lacks variety in excercises they're looking for, its user interface is not as well suited to their workouts as they would like, it was a lot different than they were expecting, or they're really not motivated to excercise. Some of these things are a perfect opportunity for third party publishers to step up to the plate ...
You can make excercises more customizeable, allowing for true beginners and fitness experts to both take advantage of your software; you can "Steal" the RockBand/Guitar Hero business model and sell excercise packs which creates endless variety; and you can fix the user interface and allow users to create their own workout curcits which (greatly) reduces the ammount of time it takes to get a large play time.
