Because there would be no way at all of scaling the tax based on income / wealth? So someone earning almost nothing with no wealth would be expected to pay the same tax as a billionaire?
This also seems bad: "The proposed Fair Tax Act would apply a tax, once, at the point of purchase on all new goods and services for personal consumption" - "personal consumption" is arbitrary. A VAT tax applied everywhere for all goods would be fair and harder to avoid. Though even that seems like it would encourage financialization to hide material production.