Because people who are retired and living off savings will all of a sudden have 20% less purchasing power. They will be paying tax on their already taxed savings. This is why the fairtax will never get the old folk vote and will never pass.
ARe you one of those who will lose some purchasing power in your retirement savings or one who will reap a financial windfall. If you have a lot of retirement savings it is likely you fall into one of those two categories. For that reason, it shouldn't be too great a leap for a one time adjustment between those two extremes. I'm sure it can be addressed in congress.
I guess the fact that the FiarTaxers rail against double taxation while creating a scheme that double taxes savings for a class of Americans is irrelevant. Just a minor transitional cost that others will bare. No big deal.