I'm sure that's the main reason. High trading power is a must for those wanting to get into DVC. It isn't enough to be a 5-star resort, but to get good trading power it must be in a very desirable location also, and then must also have desirable dates. For example, someone who owns in Hawaii, such as a Hilton, Marriott, or Hyatt, is going to have extremely high trading power. Big ski resorts such as in Vail, Breckenrdige, etc also get high trading power (provided the trade is during ski season. During the summer months these types of resorts would have much lower trading power.)
The thing is that most DVC owners don't trade at all. And those who do like to trade out to other resorts, many of them own other timeshares besides Disney, and use those other timeshares to trade out. For example, we also own Marriott in Palm Springs. Whenever we trade, we trade that resort. We never trade DVC.