I think the economy is somewhat responsible but I also think many do not understand DVC until after they buy.
For one, it is not the least expensive way to see Disney World. Also it can be restrictive for those that can not plan a trip 11 or 7 months out.
Dues can become a problem if there is a loss or reduction in income.
Of course it's not the least expensive, but noi everyone wants to stay off site or in a value resort. Some recent promotional offers have also been cheaper, as have been some point rental opportunities, but neither is a "sure bet" (at those levels) for the future. And, depending on the desired accommodations, these other options also require planning quite far in advance. DVC is all about long-term vacation planning, but it also allows for spontaneous trips and the basic cost is a known quantity.
As with any long term commitment including other timeshares, the responsibility can become a burden if income levels change. However, I suspect DVC has more options than many other timeshares, such as selling (at a better value than many other timeshares) or renting points more easily/at a better rate.